METALLO-PORPHYRIN CONTAINING ZIRCONIUM PHOSPHONATE
THIN FILMS: STRUCTURE AND CATALYSIS
.• ^ \.v ■■*■>..
CHRISTINE MARIE NIXON LEE
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
First, I would like to thank some of the teachers who have pointed me toward
chemistry and supported me in this long educational adventure. I thank Mr. Roger
Craig from Lexington High School, for being so excited about chemistry, and Dr. Jim
McCargar, for being a tireless ambassador of general and physical chemistry, and for
encouraging (pushing?) me to pursue research opportunities outside of Baldwin-
Wallace College. I would also like to thank Dr. Gary Kosloski, who cared about my
music even after I defected to the other half of the liberal arts and sciences. And to
Dr. John West and Dr. William Samuels, I extend many thanks for accepting me into
their research programs and giving me two unique and valuable perspectives on
research outside of the academic envirorunent.
I must thank Dr. Dan Talham for his patience, the limits of which, I have
surely tested. I would also like to thank him for accepting a physical polymer
chemistry convert and for teaching me to appreciate materials and surface chemistry.
I would also like to thank him for his time and effort in encouraging all of us to be
organized and effective speakers and writers. For that, I will be eternally grateful.
I could not have completed this document or the research that it describes
without the collaborative assistance from the Bujoli group in Nantes, France.
Especially to Fabrice Odobel, who has contributed significant time and energy to
making and teaching others to make the porphyrins and ligands on which this
dissertation is focused, Merci!
Quick notes of thanks are also due to the Butler Polymer Research
Laboratories for sharing their instrumentation and to Eric Lambers and the Major
Analytical Instrumentation Center for allowing me to use the XPS. Also, thanks to
Joe and Raymond in the chemistry department machine shop for working with me on
designing and building the catalysis flow cells.
Without many wonderful friends, my time at University of Florida would have
been much less enjoyable. So, I thank Louarm Troutman, Tracey Hawkins, Dean and
Annie Welsh, Denise Main, and Debby Tindall, and of course, Jen Batten and Leroy
Kloeppner (and Alex) - the greatest of friends. I owe Jen and Leroy special thanks,
not only for their efforts in editing this dissertation, but also for their support and love
along the way.
And lastly, my family. All the gratitude in the world goes to my mom and dad,
Pat and Ron, and to my brothers Joe and John and my soon-to-be sister, JuUia. And to
my grandmothers, Evelyn Nixon and Margaret Loeber, and in memory of my
Grandpas Bill and Lee. I have been truly blessed. Thanks, too, to my new family,
George and Agnes, Brian and Greg Lee who are the best second family I could
To my husband, Larry, my best friend and biggest fan, I owe more thanks than
can be expressed.
TABLE OF CONTENTS
LIST OF TABLES vii
LIST OF FIGURES viii
1 INTRODUCTION 1
1.1 Ultrathin Films 1
1.1.1 Langmuir-Blodgett Films and Characterization 3
1.1.2 Self-assembled Films 15
1 .2 Hybrid Organic/Inorganic Ultrathin Films Based on Layered Solids 16
1.2.1 Background 16
1 .2.2 Self-assembled Films Incorporating Metal Phosphonate
1 .2.3 Metal Phosphonate Langmuir-Blodgett Films 20
1 .2.4 Dual-Function Langmuir-Blodgett Films 23
1.3 Background on Porphyrins 24
1.3.1 Optical Behavior of Porphyrins 24
1.3.2 Background on Manganese Porphyrins 33
1.3.3 Immobilization of Porphyrins 35
1.3.4 HeterocycUc Ligand Cocatalysts 36
1.4 Dissertation Overview 38
2 EXPERIMENTAL 42
2.1 Langmuir-Blodgett and Self-Assembled Film Preparation Procedure 42
2.1.1 General Langmuir-Blodgett and Self- Assembly Procedures 42
2.1.1 Characterization 46
2.2 Porphyrin Films 48
2.2.1 Palladium Porphyrin Films 48
2.2.2 Manganese Porphyrin Films 49
2.2.3 Manganese Porphyrin/Imidazole Mixed Films 51
2.3 Catalysis 53
2.3.1 Catalysis using PhIO as an oxidant 53
2.3.2 Catalysis using Peroxides as oxidants 57
PALLADIUM PORPHYRIN CONTAINING ZIRCONIUM PHOSPHONATE
THIN FILMS 58
3.1 Background on Palladium Porphyrin Films 58
3.2 Results 61
3.2.1 UV-vis of Palladium Porphyrin Solutions 61
3.2.2 Langmuir Monolayers of Palldium Porphyrins 63
3.2.3 Langmuir-Blodgett Films 69
2.3.3 Conclusions 79
MANGANESE PORPHYRIN CONTAINING ZIRCONIUM
PHOSPHONATE THIN FILMS 83
4.1 Background 83
4.2 UV-vis Behavior of MnTPPs 85
4.2.1 Solution studies 85
4.2.2 Langmuir Monolayers 93
4.2.3 Langmuir-Blodgett Films of pure MnP4 95
4.2.4 Self-assembled films of MnP4 100
4.3 Conclusions 106
INCORPORATION OF AN IMIDAZOLE LIGAND INTO MANGANESE
PORPHYRIN CONTAINING ZIRCONIUM PHOSPHONATE
THIN FILMS 109
5.1 Background 109
5.2 Solution Studies 113
5.2.1 MnPO and MnP4 with ImH 113
5.2.2 MnP4 and MnPO with ImODPA 115
5.3 Film Studies 117
5.3.1 Langmuir-B lodgett Films containing substituted MnP4 117
5.3.2 Mn-porphyrins substituted into self-assembled films of
5.3.3 Self-assembling the MnP4 and ImODPA from a
mixed solution 130
5.3.4 Other methods for preparing ImODPA and MnP4 containing
5.3.5 Characterization of films containing MnP4 and ImODPA
by XPS and ATR-IR 134
5.4 Conclusions 141
6 MANGANESE PORPHYRIN AND IMIDAZOLE CONTAINING
ZIRCONIUM PHOSPHONATE THIN FILMS AS CATALYSTS 144
6.1 Background 144
6.2 Results 147
6.2.1 Catalysis With PhIO as the Oxidant 147
6.2.2 Catalysis Using H2O2 as the Oxidant 159
6.3 Conclusions 163
BIOGRAPHICAL SKETCH 171
LIST OF TABLES
3.1 UV-vis data from symmetric and alternating films of PdP4. Xmax. is given for
monolayers, and interlayer thickness is given for multilayers of films
transferred under a variety of transfer conditions 71
3.2 UV-vis data from symmetric and alternating films of PdPl . X,max is given for
monolayers, and interlayer thickness is given for multilayers of films
transferred under a variety of transfer conditions 77
6.1 Time dependence of epoxidation of cyclooctene using 40 |amol cyclooctene
and 5 famol PhIO in ImL of solution. To the homogeneous reaction was
added InmolofMnPO 151
6.2 Conversion of cyclooctene to cycloctene oxide with 40 |amol cyclooctene
and 5 i^mol PhIO in ImL of solution using MnP4 LB film 154
6.3 Conversion of cyclooctene to cyclooctene oxide with varying cyclooctene
to PhIO ratios in ImL of solution using MnP4 SA films over 24 hr 155
6.4 Conversion of cyclooctene to cyclooctene oxide with 40 ^mol cyclooctene
and 5 ^imol PhIO in ImL of solution and in films containing imidazole 157
6.5 Comparison of blanks and homogeneous epoxidation yields in vials vs. in
the reaction cells 158
6.6 Conversion of cyclooctene to cyclooctene oxide with 400 fimol cyclooctene
and 80 ^mol H2O2 in ImL of solution using imidazole and porphyrin 161
LIST OF FIGURES
1.1 Schematic of an isotherm and corresponding monolayer behavior 5
1.2 Schematic of X-, Y-, and Z-type Langmuir-Blodgett multilayers 7
1.3 X-ray diffraction diagram 9
1.4 Illustration of ATR-IR Experiment 10
1.5 Schematic of XPS experiment H
1.6 Schematic of polarized UV-vis experimental beam directions 13
1.7 Behavior of the oblique dichroic ratio versus an orientation parameter (P) 14
1.8 Crystal structure of zirconium phosphate 17
1 .9 Comparison between tradition LB films and metal-phosphonate LB films 20
1.10 Schematic of formation of divalent or trivalent metal phosphonate films 22
1.11 Structures of porphyrin-type molecules A) porphine B) free base
porphyrin, and C) pthalocyanine 25
1.12 UV-vis spectrum of a metallo-porphyrin(PdTPP) 25
1.13 Outline of 1 6-member principle resonance structure of metallo-porphyrin 26
1.14 Gouterman's four-orbital model 27
1.15 Transition dipole moments in metallo-porphyrin 30
1.16 Porphyrin chromophore interactions: The square represents the
chromophore and its disecting axes. A) H-type or face-to-face
aggregates; B) edge-to-edge aggregates; C) J-type or head-to-tail
1.17 Suggested mechanism of olefin epoxidation catalyzed by MnTPP 35
2.1 Schematic of Langmuir-Blodgett trough and monolayer 42
2.2 Schematic of the three-step deposition process used for zirconium
phosphonate films 45
2.3 Schematic of catalysis cell, side view 54
2.4 Schematic of catalysis cell, top view 55
3.1 Structures of A) PdP4 and B) PdPl 59
3.2 Schematic of Pd-porphyrin films formed: a) alternating ODPA/Zr/PdP,
b) alternating ODPA/Zr/PdP:ODPA mixed film, c) symmetric
PdP/Zr/PdP, d) symmetric PdP:ODPA/Zr/PdP:ODPA 60
3.3 Solution UV-vis of Pd-porphyrins in CHCI3: A) PdP4, B) PdPl 62
3.4 Solution UV-vis of PdP4 in EtOH and water compared to CHCI3 63
3.5 Isotherms of PdP4, pure and mixed with ODPA (PdP4:0DPA), on a
water subphase 64
3.6 Reflectance UV-vis of PdP4 on water subphase 65
3.7 Isotherms of PdPl, pure and mixed with ODPA (PdPl :ODPA), on a
water subphase 66
3.8 Reflectance UV-vis of PdPl on water subphase 67
3.9 Reflectance UV-vis of 10% PdP4: 90% ODPA on a water subphase 68
3.10 Mean molecular area vs. ratio of ODPA/Porphyrin: A) PdP4, B) PdPl 69
3.11 Transmission UV-vis of PdP4 films transferred at high and low MMA.
Absorbance scale corresonds to the film transferred at 300 A^ molecule'' 72
3.12 UV-vis of SA PdP4 films rinsed in hot CHCI3 75
3.13 Transmission UV-vis of films of PdPl transferred at high and low MMA 76
3.14 Absorbance of Soret vs. time rinsed in hot CHCI3: A) PdP4, B) PdPl 78
3.15 Illustration of orientation and packing of PdP 1 films transferred at high and
low MMA 79
3.16 Illustration of orientation and packing of PdP4 films transferred at high and
4.1 Structures of A) MnP4 and B) MnPO 84
4.2 UV-vis of MnPO in CHCI3 87
4.3 Solvent behavior of MnP4 in water, EtOH and CHCI3 88
4.4 UV-vis concentration study of MnP4 in CHCI3: a) 10"^ M, b) 10"^ M 88
4.5 MnPO in CHCI3 (1 x lO"' M) with ethylphosphonic acid: a) pure MnPO,
b) 1 X 10"* M ethylphosphonic acid, c) 2 x 10"^ M ethylphosphonic acid,
d) 3 X 10"^ M ethylphosphonic acid, e) pure MnP4 90
4.6 Solution UV-vis investigation of MnP4's sensitivity to displacement of
R-PO(OH)^ by chloride at 1x10"^ M. The arrows indicate the changes
in the intensity of the peaks as the chloride concentration changes from
0.0 M to 0.1 M while the concentration of MnP4 stay constant in CHCI3 93
4.7 Isotherm of MnP4 on water subphase 94
4.8 Reflectance UV-vis of MnP4 on water subphase 95
4.9 UV-vis of MnP4 capping layers transferred onto ODPA/Zr at different
surface pressures (indicated by the arrows) 96
4.10 LB films of MnP4 transferred at A) 1 5 mN/m and B) 5 mN/m rinsed in
4. 1 1 MnP4 transferred by LB at 0.7 mN m'' and rinsed in CH3CN: A)
transferred from a 0.5 mg mL'' solution 99
4.12 MnP4 transferred from 0.1 M [Cf] aqueous subphase at 4 mN m"' 100
4. 1 3 MnP4 self-assembled from EtOH/H20 and rinsed in CHCI3. The legend
indicates the spectra after rinsing, after being left overnight and the
rinsed again over a three day period 101
4.14 SA MnP4 films with rinsing in hot CH3CN 102
4.15 UV-vis response of a SA MnP4 film during rinsing with hot CH3CN 103
4.16 UV-vis of MnP4 self-assembled films before and after rinsing in hot EtOH 104
4.17 MnP4 self-assembled from a 0.1 M chloride solution 105
4. 1 8 XPS of MnP4 SA film. The insert is an enlarged view of the same spectrum
between 200 and 80 eV 106
5.1 Structures of A) MnP4, B) MnPO, C) ImODPA and D) ImH 110
5.2 Simplified Schematic of MnP4 and ImODPA incorporation in films 11 1
5.3 Solvent response of A) MnPO and B) MnP4 to ImH 114
5.4 Solvent response of A) MnPO and B) MnP4 to ImODPA. Legends indicate
the molar ratio of MnP to ImODPA 116
5.5 UV-vis of ODPA/Zr/HDPA, SA MnP4 film rinsed in hot CHCI3 119
5.6 MnP4 substituted onto ImODPA:HDPA LB films after CHCI3 rinsing 121
5.7 MnP4 substituted onto a 25% ImODPA/HDPA film, rinsed in room
temperature and hot CHCI3 122
5.8 UV-vis of an ImODPA/ MnP4 film after drying 123
5 .9 MnPO attached to a 25% ImODPA/HDPA LB film and rinsed in hot CHCl 1 24
5.10 MnP4 substituted onto a pure ImODPA SA film 126
5.11 Reversibility ofthechloride/phosphonic acid binding 127
5.12 MnP4 substituted film rinsed in chloride and t-butylamine solutions 128
5.13 MnP4 substituted from a 0. 1 M CI- solution onto an ImODPA layer, and
compared to an MnPO solution with ImH binding 129
5.14 ImODPA/MnP4 self-assembled from 70/30 mixture and rinsed in hot
5.15 ImODPA substituted into a MnP4 LB film transferred at 10 mN m"' 132
5.16 LB film of MnP4/ImODPA transferred from a 25/75 mixture on an aqueous
subphase, pH 11.3 I33
5.17 XPS muhiplex scan over the Nis region of A) ImODPA, and B) MnP4 self-
assembled films. The dashed line represents the Gaussian peak fit 134
5.18 XPS multiplex scan of Nl s region of ImODPA/MnP4 film self-assembled
out of 70/30 CH2CI2 solution. The dashed lines represent the Gaussian
peak fits 136
5.19 XPS multiplex scan of ImODPA/MnP4 film self-assembled from a 70/30
mixture in EtOH/HiO 136
5.20 ATR-IR of ImODPA SA film 138
5.21 Increase in absorbance intensity of 29 1 8 cm"' peak in ImODPA with
5.22 ATR-IR of alkyl region of: A) MnP4 substituted on a 1 00% ImODPA base
capping layer, B) MnP4 substituted on a 25% ImODPA base
capping layer 141
6.1 SA MnP4 film before and after 24 hr. catalysis run with 40:5:20
cyclooctene: PhIO:decane in CH2CI2 148
6.2 SA MnP4 film before and after 2 hr. catalysis run with 40:5:20
cyclooctene: PhIO:decane in CH2CI2 149
6.3 UV-vis of MnP4 film SA from chloride containing solution used in
catalysis with PhlO after 6 hr 150
6.4 Bleaching of MnPO in homogeneous catalysis reaction with PhIO 152
6.5 MnP4 LB film before and after 24 hr catalysis reaction 153
6.6 SA ImODPA/SA MnP4 studied with PhIO for epoxidation of cyclooctene 156
6.7 SA ImODPA/SA MnP4 studied in the epoxidation of cyclooctene
using H2O2 159
6.8 SA ImODPA/SA MnP4 after rinsing and after 24 hr in catalysis reaction
with excess H2O2 161
6.9 SA ImODPA/SA MnP4 film with catalysis using 8 ^mol cyclooctene
to 0.2 ^imol H2O2 162
Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
METALLO-PORPHYRIN CONTAINING ZIRCONIUM PHOSPHONATE
THIN FILMS: STRUCTURE AND CATALYSIS
Christine Marie Nixon Lee
Chairperson: Daniel R. Talham
Major Department: Chemistry
Thin films containing mono- and tetra-phosphonic acid palladium tetraphenyl
porphyrins and tetra-phosphonic acid manganese tetraphenyl porphyrins, PdPl, PdP4,
and MnP4, were prepared by both Langmuir-Blodgett (LB) and self-assembly (SA)
techniques. Within the hydrophilic regions of these films was incorporated a
zirconium phosphonate network which lent significant stability and flexibility of
preparation to these films.
In the LB films of the palladium porphyrins, it was found that the mono-
phosphonic acid porphyrins aggregated under all film preparation conditions and in
solutions at high concentrations. However, the chromophore aggregation could be
controlled in the tetra-phosphonic acid porphyrins when the films were transferred at
mean molecular areas greater or near the mean molecular area of the chromophore
itself Self-assembling the PdP4 was another means of controlling the chromophore
interaction in the films. Because chromophore aggregation was expected to inhibit
catalysis, film preparation conditions were sought in order to avoid this.
LB and SA films of pure manganese porphyrins were successfiilly prepared by
a number of different methods. Aggregation appeared insignificant when transferred
at high mean molecular areas and when the films were self-assembled. The pure
manganese porphyrin films were successfiil at catalyzing the epoxidation of
cyclooctene using iodosylbenzene as the oxidant.
To activate the porphyrin for catalysis with peroxide oxidants, a heterocyclic
ligand was also incorporated into the manganese porphyrin containing films. The
heterocyclic ligand used was an imidazole substituted with an octadecylphosphonic
acid chain. Both the manganese porphyrin and the imidazole amphiphile were
tethered to a zirconium phosphonate network, first for ease of film synthesis and
second, to stabilize the film for use in the catalysis reactions.
Though significant catalyst degradation has been reported in homogeneous and
alternative heterogeneous catalysis studies, the manganese porphyrin and imidazole
containing zirconium phosphonate films were generally more resistant to degradation
under catalysis conditions. The stability of the films toward epoxidation conditions
has led to easily recyclable catalysts.
1.1 Ultrathin Films
The study of ultrathin films, especially monomolecular thick films, enables the
study of two-dimensional systems and allows the simplification of complicated
thermodynamic behaviors. Recent interest in monolayers and multilayers focuses on
the many potential applications of organized and functional thin films, which include
optoelectronics, '-3 coatings,'*-'^ chemical sensors,' '6.8-10 and heterogeneous
catalysts.' i-'3 In order to prepare these organized and essentially two-dimensional
structures, the Langmuir-Blodgett (LB) and self-assembly (SA) techniques have been
The study of monolayer thick films began long before the early twentieth
century investigations of Irving Langmuir and Katharine Blodgett. It is believed that
centuries ago drops of oil were used to calm waves in ponds and other small bodies of
water.14,15 Benjamin Franklin studied monolayers of oil on the surface of a pond.
Agnes Pockels pioneered the study of a monolayer on the water surface in the
laboratory environment, and is credited with building the first trough. '5 However, the
first systematic study of monolayers of amphiphilic molecules on aqueous surfaces
began with Irving Langmuir's studies at GE Laboratories and, hence his name is
associated with a fundamental method of preparing organized, monomolecular thick
IT J •'
films. 16 His associate, Katherine Blodgett first transferred these films fi-om the water
surface onto solid supports. •'^
The SA technique was first described in scientific reports in the late 1940's and
mid 1950's.>8,i9 x^jg technique relies on surface-acfive molecules and the appropriate
surface being placed in contact with one another through a solvent medium. The films
prepared by this method tend to be more stable than traditional LB films because of
the types of surface interactions that drive their formation.
The traditional LB technique employs different surface interactions depending
on the method of transfer. First, as in the case of hydrophilic-hydrophilic transfers of
neutral amphiphiles from a pure aqueous subphase, the interactions are primarily
hydrogen bonding in nature. Ionic bonding is commonly observed in the case of
hydrophilic-hydrophilic transfers from a metal ion-containing subphase. In the case of
hydrophobic-hydrophobic transfers, van der Waals interactions are involved. During
LB depositions, the film is usually physisorbed to the surface implying some through-
space interaction, while in SA films, the molecules are adsorbed to the surface through
a chemical bond. The SA method relies on the formation of covalent bonds between
the surface-active component of the molecule and the solid substrate often resulting in
more stable films.^o
In LB films, the hydrophilic head group typically dictates the area the molecule
fills in the interfacial region. The alkyl groups, therefore, adjust to maximize the van
der Waals contacts leading to organized packing within this region.^' Unfortunately,
control over the film packing and organization achieved by the LB technique is absent
in the SA method. The molecular organization in SAMs (self-assembled monolayers)
is dictated solely by the geometry of the active sites on the surface. However, in LB
films, the pressure and area of deposition can be selected to deposit a particular phase
of the monolayer and to influence the transferred film organization. An understanding
of the mechanics of both the LB and SA processes is important in appreciating thin
film research and its appUcation to many areas of science.20
1.1.1 Langmuir-Blodgett Films and Characterization
18.104.22.168 Langmuir monolayer formation: the isotherm. The Langmuir
monolayer is achieved by placing droplets of an amphiphile solution, in a volatile
solvent such as CHCI3, on the aqueous subphase in such a way as to uniformly spread
the compound on the surface. Typically, the presence of an amphiphile works to
decrease the surface tension, and the difference is defined as the surface pressure. In
Equation 1.1, n is the two-dimensional surface pressure typically measured in mN m',
Yo is the initial surface tension of the subphase and Yf is the surface tension of the
subphase and film.22.23
n = r„-r/ (1.1)
The record of the monolayer formation on the water surface, the n vs. area (A)
isotherm, depends on the change in the surface pressure with a change in the mean
area per molecule (MMA) on the surface. '^'22 jhe surface pressure is measured using
a sensitive microbalance, such as a Wilhelmy balance, while the area is computer
controlled using movable barriers, which define the monolayer boundaries.
Upon room temperature spreading, the monolayer is at a very low density and
behaves like a two-dimensional gaseous phase (Figure 1.1). In this phase, the
molecules are theoretically not in constant contact with one another, although
aggregation may occur depending on the affinity of the amphiphilic molecules for one
another. There are random collisions, but there is no appreciable increase in surface
pressure because, in effect, there is not a true monolayer, and the presence of the
amphiphile has virtually no effect on the surface tension of the subphase.20 There is
some debate on the existence of a two-dimensional gaseous state; some researchers
claim that there is always aggregates formed, which interact within the gaseous
As the monolayer is compressed, the molecules will come in contact with one
another, and an observable rise in n occurs. In the initial region of noticeable pressure
increase, the molecules are colliding, but the film is in a fluid-like state. The
molecules have no long-range orientational order and they are not close-packed or
organized. This region is often called the liquid-expanded state (LE) (Figure 1.1).
Within the LE phase, the alkyl chains have many degrees of freedom, and gauche
conformations are observed within the chains.
The further compression of long chain fatty acid amphiphiles leads to a more
crystalline and organized monolayer. In this phase, the hydrophobic tails of the
amphiphiles adopt an overall orientational order that is maintained within the film
domains. This orientational order seeks to balance the van der Waals interactions
between the alkyl chains with the pressure applied by the barriers. Within this region,
referred to as the liquid-condensed phase (LC) (Figure 1.1), the slope of the isotherm
is very steep, meaning that the pressure goes up rapidly without much change in the
MMA. In some cases, the structure of the amphiphiles may prohibit the formation of a
close-packed monolayer regardless of the pressure, and therefore, the monolayer enters
and remains in the LE phase. ' ^ '•
When the monolayer cannot compress any further, additional applied pressure
from the barriers causes the monolayer to fold either over or under itself forming
collapsed regions. The collapse point is identified as the first point of deviation from
the linear slope of the LC region of the isotherm. '5.22
C. Liquid Condensed State
A. Gaseous State
MMA (A^ molecule"^)
Figure 1.1: Schematic of an isotherm and corresponding monolayer behavior.
22.214.171.124 Langmuir monolayer characterization: creep and hysterisis tests and
reflectance spectroscopy. Although the isotherm is very important in identifying the
general behavior of a monolayer film, it does not give specific information about such
things as the monolayer stability, alkyl chain orientation, or aggregation of the
amphiphiles. These more specific ideas of monolayer behavior can be discerned from
experiments such as creep and hysterisis tests, reflectance UV-vis, fluorescence
microscopy, Brewster angle microscopy, and surface potential measurements,^^ to
name a few. 15.22 x^e methods applying to the films discussed in this dissertation will
be described here.
Creep tests provide information on monolayer quality and can be recorded in
two ways. First, the monolayer is compressed to a certain pressure and then that
pressure is maintained by the barriers expanding or compressing as necessary while
recording the change in area with time. Second, the monolayer is compressed to a
defined area, which is maintained by stationary barriers, and the pressure is monitored
over time. Stable monolayers will show a static surface pressure or little movement in
the barriers after the desired pressure is reached. Unstable monolayers go through
constant and sometimes drastic rearrangements, which force contraction or expansion
of the barriers. For example, the hydrophobic nature of the amphiphile may be
insufficient and the amphiphile may dissolve into the subphase forcing the barriers to
compress to maintain FI. Also, the vapor pressure of the amphiphile may lead to their
evaporation, causing the surface pressure at constant area to decrease or the barriers to
move forward to hold constant pressure in proportion to the instability of the film.
Alternatively, the amphiphiles may have a strong affinity for one another, leading to
agglomeration and either causing an anomalous change in the surface pressure at
constant area or forcing the barriers to work to maintain the surface pressure.22
Hysterisis studies monitor the effect of the monolayer stability on the
reproducibility of the isotherm upon compression and decompression. If the
amphiphiles tend to aggregate, the isotherm will not retrace its compression curve in
its decompression cycle. From creep tests and hysterisis experiments, the ability of
the monolayer to hold its form and to possibly be transferred can be ascertained.24
Reflectance UV-vis spectroscopy is used to understand the optical behavior of
monolayers of chromophore-containing amphiphiles on the water surface. One
method for studying the Langmuir monolayer by reflectance UV-vis involves placing
a mirror on the base of the trough and reflecting a beam through the monolayer onto
this mirror, and back into a detector. These studies can help determine the onset of
aggregation in films with such a tendency.25.26
>.1.1.3 Langmuir-Blodgett film formation. LB films are formed by vertically
transferring Langmuir monolayers from the water surface onto a solid support. There
are three common vertical dipping techniques, which form X, Y, and Z-type films
(Figure 1 .2). 15-23 In X-type LB films, a Langmuir monolayer is transferred onto a
hydrophobic substrate in such a way as to maintain head to tail type interactions. In Z-
type LB films, the monolayer is transferred onto a hydrophilic substrate also forming
head to tail interactions. X- and Z-type films can be prepared on a specially designed
trough, which allows one stroke to be made through a monolayer and the next to be
made through a clean subphase. However, some amphiphiles have a preference for
this type of interaction and upon regular dipping, these structures form spontaneously.
Y-type multilayers are most common and can be prepared on either hydrophilic or
hydrophobic substrates. Y-type multilayers are typically the most stable due to the
strength of the head-head, tail-tail interactions. '^
• • •
• • •
• • •
• • •
• • •
• • •
• • •
• • •
• • •
• • •
• • •
• • •
Figure 1.2: Schematic of X-, Y-, and Z-type Langmuir-Blodgett multilayers.
1 ■ 1 ■ 1 .4 Langmuir-Blodgett film characterization. The quality of the transferred
film is first indicated by the transfer ratio, which is a measure of the change in the area
of the monolayer versus the area of the substrate coated by the monolayer. A transfer
ratio of unity indicates that the monolayer is transferred with the same area per
molecule that it had on the water surface. This "perfect" transfer ratio assumes that the
monolayer on the water surface was stable and was not reorganizing significantly
during transfer. A consistent deviation from unity could imply a change in
organization upon transfer; however, if the transfer ratio is irregular, the transferred
film is probably poor quality. ^ ^
There are many analytical techniques used to study transferred films. Film
characteristics typically of interest are thickness, interlayer spacing, molecular
orientation and packing, film coverage, surface topology, chemical composition, and
optical and magnetic properties. The techniques used to study these parameters are
well described in the literature.^o
X-ray diffraction is a reliable technique to probe interlayer spacing, and from
interlayer spacing, film thickness can be inferred. The X-rays are essentially reflected
fi-om planes of higher electron-density. The interaction of X-rays with the crystalline
planes can be described by Bragg's law (Equation 1 .2):
.^^■■:-\ nA = 2dsme (1.2)
where n is an integer. A, is the wavelength of the radiation, d is the interlayer spacing,
and is the angle of incidence and reflection of the beam.27
Ideally, there should be a significant difference between the electron density of
the head group and that of the hydrophobic region allowing X-ray diffraction peaks to
be observed in LB films. The d-spacing, which quantifies the periodicity between
planes of high electron density, therefore, is the measure of the distance between head
groups. This technique is very sensitive to long-range periodicity, and many narrow
00/ peaks are typically indicative of a well-defined layered architecture (Figure 1 .3).23
Figure 1.3 X-ray diffi-action diagram.
To study the chemical make-up of the surface, attenuated total reflectance
FTIR (ATR) and X-ray photoelectron spectroscopy (XPS) are employed. ATR studies
involve transferring a film onto a crystal, such as parallelogram-shaped germanium or
silicon crystal with ends cut at 45° angles. This is an ideal method for recording the IR
spectra of films because it allows the film to be sampled many times due to the
internal reflection of the IR beam through the crystal. ATR also provides information
about the surface coverage and packing modes. Further, polarized studies using this
technique can give information about the film organization and orientation (Figure
1.4). '5 Background information pertaining to films studied by ATR-IR, allows
elucidation of information about films containing new amphiphiles. For example, by
comparing the areas of the alkyl stretches of a new amphiphile to that of well-
understood fatty-acid films, an idea of the transfer quality can be obtained. This tool
is particularly helpful in studying monolayers with transfer ratios varying from unity.
ATR-FTIR can also indicate the packing nature of the alkyl chains. If the
chains are in a mostly trans configuration and close-packed, the asymmetric CH2
stretch (v^CHj) will occur at 2918 cm'' and have a full width at its half maximum
(FWHM) of ca. 20 cm-'. The symmetric CH2 stretch (v.CHj) will occur at ca. 2852
cm-'. If there are a significant number of gauche interactions in the chains, the V3CH2
and V5CH2 stretches will shift to higher energies (ca. 2924 cm-' for the v^CHj and ca.
2856 cm-i for the v.CH^ stretches).28.29
Figure 1.4: Schematic of the ATR-IR Experiment.
XPS is a method used to determine the elemental make-up, and possibly,
atomic proportions within a film based on the photoelectron effect. XPS measures the
energy of an expelled electron as the surface is bombarded with a monochromatic X-
radiation source (Figure 1.5). When a high-energy source is applied, the kinetic energy
of the emitted electrons can be related to the excess energy and to the strength of the
electron's binding by Equation 1.3, which is the Einstein photoelectric law:27
where Et is the binding energy, e is the charge of the electron, h is Planck's constant, v
is the frequency of the radiation, and (f) is the work function corresponding to the
minimum energy required for ejection of an electron.
Figure 1.5: Schematic of XPS experiment.
The XPS spectrum plots electron counts versus binding energy. Binding
energies are unique to each element present in the film, as well as to that element's
chemical enviromnent and oxidation state. Therefore, from a scan over a wide range
of binding energies, called a survey scan, XPS results can be used to define the
elements present in the sample. A more extensive scan, or a multiplex scan, over a
narrow range of binding energies can clarify peak splitting, which can be assigned to a
change in the chemical environment or oxidation state. Offord, et. al., in their study of
a Ru-porphyrin linked to a thiol on gold SA film containing a percentage of imidazole
terminated thiol surfactants, observed two peaks within the N,, region of the XPS
spectrum. These two peaks were associated with the different nitrogen environments
in the imidazole and porphyrin, clearly indicating that both species were present in the
mixed film. 30
The intensities of the XPS peaks can be used to determine the relative ratios of
the elements present, and can indicate the type of crystalline lattice formed. However,
the intensities of the XPS peaks are sensitive to many parameters such as the element's
electron escape depth, which can complicate the determination of the elemental ratios.
In a given sample, the observed relative peak intensities are compared to a calculated
value based on Equation 1.4:
where I^ is the relative intensity of element A, 1^°° is the atomic sensitivity factor, d„ is
the overlayer thickness, is the incident angle of the X-ray beam, and X^ is the
inelastic mean free path. The inelastic mean free path represents the distance over
which 60% of the electrons can travel before inter-electron collisions lead to a loss of
energy3i and is defined by :32
A,=10[49/(£^,/) + 0.11(£,„,r](A)
UV-vis spectroscopy reveals a film's optical behavior. Typically, films are
transferred onto glass or quartz substrates and transmittance studies are performed.
Using polarizers and stages that allow careful placement of the substrates at different
angles of incidence to the beam, the orientation of the chromophores within the films
can be determined. By comparing the absorbance intensity in the s- and p-
polarization, a dichroic ratio can be calculated using Equation 1 .6:
Orientational order within the plane of the film and substrate is obtained by looking at
results from 0° and 90° (s and p) polarization as the beam is normal to the surface (0°
angle of incidence). Studying the polarization at higher angles of incidence (typically
45°) enables the determination of the orientation out of the plane (Figure 1 .6).
\ \V' ' Qo polarization
Figure 1.6: Schematic of polarized UV-vis experimental beam directions.
The dichroic ratio can be used to determine the chromophore orientation in a
film. In the case of films containing porphyrin chromophores, Orrit et al. determined
that the dichroic ratio can be translated into an orientation parameter (P) using the
graph shown in Figure 1.7,25 and hence, an orientation angle (6) can be established
using Equation 1.7:
P = (cos^0)
represents the angel between the surface normal and the chromophore's molecular
plane. As is often observed within the plane of a film containing porphyrins, if D is
unity, there is no preferred orientation. If D = 1 .5, P = and therefore, 9 = 90 ° as
measured from the surface normal.
0.2 0.4 0.6 0.8
Orientation Parameter (P)
Figure 1.7: Behavior of the oblique dichroic ratio versus an orientation parameter
Porphyrin containing films often have oblique dichroic ratios of approximately
1.5, corresponding to the chromophores lying parallel to the substrate. For example,
Zhang et al., obtained such a result in LB films containing a free base tetraphenyl
porphyrin either pure or mixed with stearic acid, where the porphyrin was the
hydrophilic head group with long alkyl substituents.33
1.1.2. Self-Assembled Films
Zisman introduced SAMs to the literature little more than 50 years ago. His
studies involved self-assembling long-chained alcohols onto a glass surface using
hexadecane as the inert solvent. 18 This study showed that films prepared in this
manner had wetting properties similar to those seen in films prepared by the LB
Sagiv et al. studied octadecyltrichlorosilane (OTS) on hydroxylated surfaces,
such as glass, to form a siloxane polymer. Multilayers were produced if the
amphiphile was terminated at both a and co positions by surface-active groups. 15,34
When a and co positions were two different ftinctionalities, subsequent dipping in self-
assembly solutions produced non-centrosymmetric films. Unfortunately, studies have
shown that small defects in the early layers can magnify upon multilayer formation
such that nearly all order breaks down by the tenth layer.20 The SA of OTS is now
commonly used for hydrophobisizing glass slides for LB substrates.
The chemisorption of thiols on gold was initiated by Nuzzo and AUara^s and
continued by Porter,36 Whitesides,37 and others. Alkyl thiols, in which the chain
lengths range from one carbon to over twenty, have been studied. Closely packed
layers were observed when the chain length exceeded eleven carbons.36 The gold
surface used in these studies was formed by vacuum evaporation onto cleaved alkali-
metal halide surfaces.37 Many of the surfactants studied by self-assembly did not form
stable monolayers on aqueous subphases, and therefore, could not be studied by the
LB method. However, these surfactants were easily studied by the SA method.
1.2. Hybrid Organic/Inorganic Ultrathin Films Based on Layered Solids
A new class of LB films has been developed, which incorporate an inorganic
metal phosphonate network into the polar region.28.38-4i These films, which can
contain a variety of organic groups and metals, were inspired by and show analogous
behavior to their solid-state metal phosphonate analogues. In addition, these films are
more stable than typical fatty-acid based LB films, and the inorganic lattice provides
The metal phosphonate solid-state materials are attractive because they can be
prepared at low temperatures from aqueous solutions. Further, the structures can
provide a model system to which the film properties can be compared. Due to the
structure of the metal lattices, which directs the film formation, the orientation and
packing of the alkyl region is predictable.
Metal phosphonate materials are especially interesting due to their potential
applications in the areas of sorbents43 and catalysts,44 and because of their layered
structures, these materials can be used as intercalation compounds.45-49 x^g interest in
the metal phosphonates was sparked by their potential as inorganic ion exchange
materials;50-52 however, the organic region can also be modified and functionalized,
providing a straightforward method for preparing a wide variety of materials.
1 .2. 1 . 1 ■ Zirconium Phosphonate solids. Clearfield published early work on
metal phosphonates and phosphates in the 1960's.53 The focus at this time was on the
zirconium solids which form two preferred phases, a and y, which have the
compositions Zr(HP04)2-H20 and Zr(P04)(H2P04)-2H20, respectively. In these
layered materials, a two-dimensional metal lattice is formed and separated from an
adjacent metal lattice by the organic layer in the phosphonates or by the hydrogen
bonds from the fourth hydroxy site in the phosphates. The interlayer area in these
solids forms a possible domain for intercalation of inorganic materials such as
Figure 1.8: Crystal structure of zirconium phosphate.53
The crystal structure of the Zr-phenylphosphonate solid was determined in the
1 980's and found to form a structure similar to the a-phase. Subsequent studies
revealed that any alkyl or aryl group whose area was under 24 A2 would form an
identical metal lattice structure while only the interlayer distance changed. In the n-
alkyl phosphonates, it was found that there is a tilt angle in the chains of between 55°
and 60°. This tilt allows for maximization of the van der Waals forces between the
chains even within the solid-state materials. Though the structures formed are dictated
by the geometry of the inorganic lattice, the hydrophobic region does rearrange to
balance these strong forces with the maximization of their overlap.53 A change in the
organic group can lead to very different structures and properties of the solids. Bulkier
groups may form new crystal structures or have three-dimensional metal lattice
formation. Also, the different organic groups can impart different function to the
1 .2. 1 .2. Divalent and trivalent metal phosphonate solids. After extensive
research on the zirconium phosphonate solids, interest branched to divalent and
trivalent metal phosphonates. The poor solubility of the zirconium phosphonate solids
in most standard solvents meant achieving single crystals was difficult; therefore, most
of the crystal structure data was achieved from powder X-ray diffi-action patterns.
However, di- and trivalent metals tend to be soluble in acidic solutions, allowing
single crystals to be obtained by slowly changing the solvent pH or the metal ion
Metal phosphonate materials have been prepared with a variety of different
divalent metals such as Mg^\ Mn^^ Zv^\ Q,^\ and Cd^^54-56 prom the crystal
structures, it was determined that the composition of the divalent series metal
phosphonates is M"(03PR)-H20 for Mg, Mn, Zn, Ca and Cd. In these materials,
layers of the metal atoms are octahedrally coordinated by five phosphonate oxygens
and one water molecule, with each phosphonate group coordinating four metal atoms
making a cross-linked M-0 network. A second structure, the orthorhombic
M"(H03PR)2, was observed for the Ca phosphonates. A structural exception to the
above divalent series is Cu(II).''8.57 The Cu atoms in these phosphonate materials are
five coordinate and form a distorted tetragonal pyramidal geometry.
Mallouk has prepared a series of lanthanide phosphonates. The structure of
these materials is given as Ln(III)H(03PR)2, where Ln represents La, Sm, or Ce. The
lanthanide-series phosphonates are more soluble than the zirconium solids, but less
soluble than the divalent materials. Therefore, single crystal data was not easily
ATR-IR provides a facile method for characterization of the metal phosphonate
lattice formation. Vibrational modes assigned to the phosphonate are extremely
sensitive to the mode of metal binding. Thomas et al. have assigned the VJCH2)
vibrational frequencies for the divalent metal-phosphonates to be in the range of ca.
1050 - 1 100 cm-' where the V3(CH2) stretches occur ca. 970 - 990 cm-'.60 Each metal
phosphonate material has characteristic stretches in these regions.
1.2.2. Sel f-Assembled Films Incorporating Metal Phosphonate Binding
After Sagiv's work with self-assembling films of octadecyltrichlorosilane
(OTS), it seemed a natural step to translate the formation of the thermodynamically
stable and insoluble layered metal phosphonate solids into ultrathin films. Self-
assembly was the first technique employed to produce metal phosphonate thin films.
Mallouk and coworkers formed the metal phosphonate self-assembled films by first,
exposing a silicon or gold surface to an appropriate template forming alkyl-mercaptan
that was substituted with a terminal phosphonic acid.6i.62 This phosphonic acid was
active toward the metal salt solution in which the substrate was then dipped. After
metallating the surface, the substrate was dipped in a solution of an a, co-
bisphosphonic acid, which left another phosphonic acid on the surface to be
subsequently metallated, and the cycle continued until multilayered films were
The self-assembly of metal phosphonate films is made possible by a very
strong attraction between certain metal ions, particularly the tetravalent metals such as
Zx'^, for the phosphonate groups of alkyl phosphonic acids.6i-63 However, the
individual metal salts and the phosphonate are themselves soluble. This particular
affinity between metal and phosphonate, makes possible the formation of
monomolecular layers during each step of the cycle and the ability to assemble
1.2.3. Metal Phosphonate Lanemuir-Blodgett Films
Two methods of film formation have been employed to incorporate the metal
phosphonate lattice into the polar region of LB films, one for the divalent and trivalent
metals which are soluble in acidic media, and one for the tetravalent and some
trivalent metals which are insoluble even at low pH. A schematic comparing
traditional LB films to metal-phosphonate LB films is shown in Figure 1 .9.
A ^^A^^ region ^ i^i^^^^
' ' I ' I phosphonate I I | | |
Figure 1.9: Comparison between A) traditional LB films and B) metal-phosphonate
The zirconium metals have such a high oxophilicity for the phosphonate
oxygens, that when a phosphonic acid amphiphile is spread on the surface of a
zirconium cation containing aqueous subphase, the monolayer crystallizes before it
can be transferred. Therefore, a three-step deposition procedure has been developed.
A phosphonic acid containing monolayer is formed on the surface of a pure water
subphase and transferred onto a hydrophobic substrate. Onto this phosphonic acid
surface, a layer of zirconium is assembled, followed by the transfer of a second LB
monolayer containing a phosphonic acid. This three-step deposition technique will be
described in detail in Chapter 2.28.38
Advantages of this three-step technique include the fact that at the hydrophilic
stage, the monolayer is stable and can be independently characterized by ATR-FTIR
or XPS, etc. Second, this method allows the formation of alternating films in which
the template and capping layers do not have to be formed of the same amphiphile. The
option of forming alternating films is important because some amphiphiles do not
transfer on the down stroke but will transfer onto an ODPA template layer. A
disadvantage of the three-step deposition of the phosphonate films occurs at the self-
assembly of the zirconium lattice. The self-assembly of the zirconium onto the ODPA
template causes the metal phosphonate lattice to be amorphous, whereas in a one-step
deposition, the metal lattice is crystalline.
An alternative technique of film formation is employed for the divalent and
trivalent metals.''! In these cases, the metal salts are dissolved in the aqueous
subphase, the phosphonic acid monolayer is formed on the surface, and the
hydrophobic substrate is dipped dovm and then up through the same compressed
monolayer forming a complete metal-phosphonate layer or an LB bilayer. The
crystallization of the metal-phosphonate lattice occurs on the slow upstroke of the film
through the monolayer (Figure 1.10).
In the one-step method, the pH of the subphase is as crucial to successful
lattice formation as it is in the formation of the solids in aqueous solutions. If the pH
is too high, the affinity of the metals for the deprotonated phosphonates will be too
high and crystallization of the lattice will occur in the Langmuir monolayer rather than
upon transfer. As with the zirconium films, these films will be too rigid to be
• ^ • »■
.• « • .
e • ^
Figure 1.10: Schematic of formation of divalent or trivalent metal phosphonate films.
If the pH is too low, the phosphonate will remain completely protonated, and the films
will transfer without metal binding. Fortunately, the pH effects on the crystallization
of the monolayer have signatures in the isotherm behavior.64 When the pH is too high,
the rigid monolayer gives an erroneous but characteristic isotherm that has a much
higher onset and shallower incline. We believe this isotherm behavior is due to the
rigid films causing deflections in the Wilhelmy balance rather than showing an
increase in surface pressure.
1 .2.4. Dual-Function Langmuir-Blodgett Films
After the extensive characterization of simple alkyl metal phosphonate LB
films, there was interest in incorporating fiinction into the organic region that might be
paired with properties in the inorganic lattice to form a "dual fimction" LB film. As
models, phenoxy and biphenoxy alkyl phosphonic acids were prepared, and divalent,
trivalent, and tetravalent metal phosphonate films were studied. 65,66 Additionally,
films containing azobenzene-derivatized phosphonic acid amphiphiles were
synthesized, and metal phosphonate films were formed also with divalent, trivalent,
and tetravalent metals.67 These results prove that larger organic groups can be
incorporated into the metal phosphonate LB films while maintaining the integrity of
the inorganic lattice structure.
Potential applications of dual functional metal-phosphonate thin films include
magnetic switches, in which the magnetic behavior of the inorganic lattice can be
altered by a structural change in the organic region. Also, films containing a
conductive or non-linear optic organic region as well as a magnetic inorganic lattice
could act as a sensor. This dissertation will focus on the preparation of porphyrin
containing zirconium phosphonate films where the metal phosphonate lattice acts to
stabilize the films toward potential catalytic reaction conditions.
1 .3 Background on Porphyrins
1.3.1 Optical Behavior of Porphyrins
Porphyrins are a common research focus in physics, chemistry, and biology.
Physical and chemical interest in porphyrins stems, for example, from their highly
conjugated structure that allows facile electron-transfer,68-72 and from their chemical
actiyity at an exposed metal that may be active toward catalysis or chemical
sensing. 1.8,73-75 Biologists and biochemists are interested in the common biological
building blocks that are based on the porphyrin structure. 76-78
The core structure of the porphyrin is the completely saturated porphine
macrocycle (Figure 1.1 1).79 Upon reducing this macrocycle to the unsaturated form,
the porphyrin chromophore is achieved. By hydrolyzing one of the pyrrole units, the
chlorin compound is prepared. Another important structure based on the porphine
core is the phthalocyanine or the tetraazatetrabenzporphyrin. Each of these structures
includes either two protons, as in the free base porphyrin, or a coordinated metal
within the center of the porphyrin, called the metallo-porphyrin. Examples of
biologically active porphyrins include chlorophyll, which is a manganese-coordinated
chlorin molecule, and heme, which is an iron-substituted porphyrin.
Figure 1.11: Structures of poiphyrin-type molecules A) porphine B) free base
porphyrin, and C) pthalocyanine.
Porphyrins have characteristic and strong optical transitions by which they can
be identified. The bands often observed in visible spectra of porphyrins include the B
or Soret Band and Q Bands, as seen in Figure 1.12 for a palladium
tetraphenylporphyrin (PdTPP). The Soret Band is associated with the allowed n-n*
transition and is typically seen between 380 and 420 nm.^o
Figure 1.12: UV-vis spectrum of a metallo-porphyrin (PdTPP).
The Q-Bands are observed between 500 and 600 nm. The lower energy Q-
Band (Q„) is associated with the electronic origin, Q(0,0) of the lower energy singlet
excited state. The higher energy Q-Band (Qp) has a contribution from a vibration
mode and is denoted Q(1,0). Both Q(0,0) and Q(1,0) are quasi-allowed transitions
with relatively low absorbance intentisties. The Q-Bands are highly sensitive to the
symmetry of the molecule. In porphyrins of 04^ symmetry such as metalloporphyrins,
or the diacidic or dibasic forms of the porphyrin, two Q Bands are observed as
pictured in Figure 1.12. The free-base porphyrin is of D2h symmetry and the
degeneracy of the Q-Bands is disrupted, splitting the Q-Bands into four peaks.^o
The above described transitions are due to the porphyrin Ti-electrons and are
7i-7t* in nature. If these transitions are unperturbed by the central substituent, the
Figure 1.13: Outline of 1 6-member principal resonance structure of metallo-
porphyrin is classified as "regular". Similarly, the emission spectra of regular
porphyrins are determined solely by the chromophore itself The above explanation of
the UV-visible behavior of porphyrins is based on the free-electron model, in which
the core of the porphyrin, the 1 6-member heterocyclic, conjugated ring behaves like a
free-electron wire (Figure 1.13). Another popular theory is Gouterman's four-orbital
model (Figure 1.14), which combines the Huckel-MO theory with the free-electron
model. In this model, Gouterman describes four orbitals, two LUMOs, c,(e ) and C2
(Cg) each with five nodes, which are degenerate in energy, and two HOMOs, b,(a2j
and b2(a,J, each with four nodes, which are not degenerate. According to the four
orbital model, the Soret Band corresponds to the transition from the lower energy a,„
orbital to the e^ orbital, giving a higher energy transition. The Q-Bands arise from the
transition from the a2u orbital, which is higher in energy giving a lower energy
Figure 1.14: Gouterman's four-orbital model.
However, there are also "irregular" porphyrins. Irregular porphyrins, typically
metalloporphyrins, are broken down into categories called hypso- and hyper-
porphyrins. In the case of irregular porphyrins, the central metal contains partially
filled shells, which introduce a possibility of metal electrons mixing with porphyrin n-
electrons. This mixing is caused by the possibility of metal to porphyrin back-binding
due to similar energies of the metals d-orbitals and the porphyrin's 7t-orbitals. The
central metal ion can lead to significant changes in the optical and emission spectra.
The metal and its oxidation state determine which category the porphyrin's optical
behavior will fall into. Also, the release of electron density from the metal to the
porphyrin enables the metal to stay co-planar with the chromophore as the effective
size of the metal is reduced.'^^
Hypsoporphyrins have central metals of groups eight through eleven with
configurations d"" where m = 6 - 9 and have filled eg(d7i) orbitals. The inclusion of
these metal ions is often associated with a bathochromic or blue shift relative to the
corresponding free base porphyrin. Common hypsoporphyrins include Ni(II), Pd(II),
and Pt(II)-porphyrins. The Ni(II) porphyrins are easily affected by basic axial ligands,
whereas Pd(II) and Pt(II) are typically four coordinate and appear insensitive to the
potential ligand envirormient ^0-8'
The second class of irregular porphyrins is called the hyperporphyrins, which
is fiirther broken down into subclasses called p-type, d-type, and pseudonormal
hyperporphyrins. Most metallo-porphyrins classified as hyperporphyrins have central
metals with easily accessible lower oxidafion states. Of these, Mn(III) and Fe(III) are
the most well studied due to their biological implications. The spectra of
hyperporphyrins exhibit the Soret and Q-Bands as before with some possible shifting.
Additional prominent absorption bands may be seen typically at higher energies
relative to the Soret Band. The hyperporphyrin spectra demonstrate the effects due to
metal-ligand charge transfer (MLCT) mixed with the porphyrin ti-tt* transitions even
within the Soret Band. The MLCT Bands can be porphyrin to metal, metal to
porphyrin, or even axial ligand to metal. Due to the spectral sensitivity to
chromophore substituents, to the metal its oxidation state, and to the nature of the axial
ligand, and the additional UV-vis Bands, hyperporphyrin spectra are much more
difficult to analyze.^0,81
The d-type hyperporphyrins include metals of groups six through eight.
Mn(III)-porphyrin, for example, is d" and S = 2, or high-spin, and is a characteristic d-
type porphyrin. In chloroform, Mn(IIl) tetraphenylporphyrin shows six peaks. An
early researcher of the Mn-porphyrins, Boucher, termed these peaks by Roman
numerals going from low to high energy. The first two peaks are in the far-red region
between 800 and 650 nm. Bands III and IV absorb in a region similar to the Q-Bands
in regular porphyrins, between 500 and 650 nm. Band V is similar to and often called
the Soret Band though this band now includes contributions from metal to ligand
mixing. Band VI is typically around 350 nm. The ratio of Bands V and VI is very
sensitive to axial ligands and ring substituents. These bands are due to porphyrin to
metal charge transfer a,„(7r), a2„(7i) to qJ^&k), which implies a necessity for one or more
vacancies in the Qj^dn) orbital of the metal and reduction potentials which are not too
Finally, pseudonormal hyperporphyrins include VO(IV), Cr(II), Mn(II),
Mo(IV), La and Ac where S ^t 0. These metals show normal absorption spectra with a
weak extra absorption possible in the far-red region. All of these metals have a
partially filled or empty eg(d7i) orbital, but charge-transfers from the porphyrin to the
metal are too high in energy to be observed in the UV-vis region. In addition, fiirther
reduction takes these metals to unstable oxidation states which makes this an even
higher energy transition and highly unlikely 80,81
In addition to intramolecular effects such as the metal, substituents, and axial
ligands, intermolecular effects such as aggregation can significantly alter the electronic
behavior of porphyrins. Aggregation in these chromophores has been described
thoroughly by Kasha's exciton theory. This theory looks at aggregation only from the
point of view of overlapping transition dipole moments (Figure 1.15) and not as
interacting 7t-systems. In metallo-porphyrins, the transition dipole moments are
equivalent due to the symmetry of the chromophore.'^6'82.83
Figure 1.15: Transition dipole moments in metallo-porphyrin.
Aggregation, according to Kasha's model, splits the excitation energy of the
monomer (E") into high and low energy components. Equation 1.8 describes the
energy dependence on aggregation by:
E" =E° +D±V
where D is the dispersion energy which is highly dependent on the change in
environment upon aggregation, and V is the exciton splitting energy. 76,83,84
' V 31
When the chromophores are interacting with the transition dipole moments
parallel, the exciton energy can be described by Equation 1 .9:
where M is the transition dipole moment, R is the center-to-center distance, N is the
number of chromophores, and a is the angle between R and M (Figure 1.16). So, if a
< 54.7°, V will be positive, and the exciton splitting will be greater and a red shift will
be observed as the transition shifts to lower energy. A red shift is observed in what are
called J-aggregates where both M^ and M^ make angles less than 54.7° with the R
vector. If a > 54.7°, V will be negative, and the exciton splitting energy will be lower
leading to a blue shift in the spectrum. When M^ and My are both greater than 54.7°
fi-om R, the aggregates are termed H-type. If a, < 54.7° and a^ > 54.7°, the spectral
components will split and part of the band will shift red and part will shift blue; this
spectral behavior is seen in edge-to-edge type aggregates. There can be combinations
and varying degrees of these types of interactions within an aggregated domain
possibly leading to complicated spectra, but in general, the optical spectra ease
identification of electronic behavior of porphyrin chromophores (Figure 1.16). 76,83,84
Figure 1.16: Porphyrin chromophore interactions: The square represents the
chromophore and its disecting axes. A) H-type or face-to-face aggregates; B) edge-to-
edge aggregates; C) J-type or head-to-tail aggregates.
The transition dipoles M^ and M^, are typically parallel to the plane of the
chromophore except when the nature of the metal in the chromophore center causes a
puckering of the ring. Therefore, polarized UV-vis experiments can easily indicate
orientational changes of the chromophore within a film (Figure 1.16).''6,83,84
Incorporation of porphyrins into LB films is currently of interest in scientific
literature. These films are designed in order to prepare selective gas-sensors, ''"^^js
photovoltaic devices,^^ electron-transfer materials,''2,86 molecular wires,^"^ and novel
heterogeneous catalyst systems.^^ However, a difficulty arises in the stability of the
samples using the "typical" LB methods of purely hydrophilic/hydrophobic
interactions or by self-assembly involving tethering through a ligand. Including a
metal-phosphonate lattice into these films should significantly improve the stability
and the applicability of these materials in ultra-thin, organized films.
Typically, LB films containing porphyrin constituents have been studied in
which the chromophore itself is the polar head group. These porphyrin films have
been successfiiUy prepared with either the molecule sufficiently diluted with a fihn
stabilizing amphiphile,33,88-90 such as stearic acid, or with long hydrophobic chains
attached to the chromophore to stabiUze the monolayer on the water surface.^'*'^'
There are significant disadvantages to this method of film preparation. First, the
chromophore is buried in the film interior on a transfer onto a hydrophilic substrate,
and commonly, the hydrophobic interactions necessary to deposit onto a hydrophobic
substrate are too weak for successful transfer. Also, the hydrophilic interactions are
typically of a hydrogen-binding nature making this a relatively weak interaction
destabilizing the film. Finally, the conditions necessary for transferring the traditional
porphyrin LB films facilitate aggregate formation, which can be detrimental in certain
applications, such as catalysis.
The aggregation, or chromophore n-n interactions, is often a consequence of
the film forming procedures. First, compression of the film on the water surface
forces the eventual overlap or tilting of the chromophores-^'^'^^-^o Also, the decreased
affinity of the derivatized chromophores for water tends to force the chromophores to
aggregate rather than to spread on the water surface.92 Understanding the molecular
orientation, aggregation, and morphology of porphyrin LB films is critical because
each is intimately linked to chromophore behavior. For example, aggregation can
significantly reduce or eliminate the efficiency of the porphyrin in catalysis''^ or the
ability of the porphyrin to bind probe molecules in a sensor.^^ Therefore, it is
desirable to find methods for forming porphyrin LB films with no aggregation.
1 .3.2. Background on Manganese Porphyrins
Biomimetic systems involving porphyrin catalysts have often been discussed in
scientific literature over the past 20 years. Manganese and iron porphyrins are
commonly studied oxidation catalysts and are prevalent elements in biological
processes.93-96 Biochemical oxidation reactions employing metallo-porphyrins
involve reversible site-specific binding of the substrate such that the substrate is within
reach of the oxygen atom on the metal. After the oxygen has been successftilly
transferred to the substrate, the product is released and the catalyst is regenerated.^^
Manganese porphyrins are probably most well known as epoxidation and
hydroxylation catalysts whether under heterogeneous or homogeneous conditions.
The manganese porphyrin catalysts can utilize a number of different oxidants such as
iodosylarenes, alkylhydroperoxides, hydrogen peroxides, and perchlorates among
others, in order to accomplish the facile oxidation of deactivated olefins, alkanes,
alcohols, ethers, and amines.^'^^'O^ a hyper- valent metal-oxo species is believed to be
the active intermediate in the oxidation process in cases such as dioxygen activation of
Cytochrome P-450, or in oxygen transfer from iodosylbenzene, peracids, or
hypochlorite oxidants. '^i Though there is some debate on the actual mechanism of the
epoxidation, there are a few possible routes (Figure 1.17). The suggested first and
rate-determining step is the formation of a charge-transfer complex. Whether the
reaction then proceeds through epoxidation or rearrangement is dependent on the
oxidation potentials of the alkenes and the oxidants, steric and electronic structures of
the reactants, and the ability of the substrates to undergo rearrangement.^'^
Porphyrins have also been studied in chiral catalysis. Lai and co-workers studied
the asymmetric aziridation of alkenes using a chiral manganese porphyrin catalyst. '02
They found that with bulky chiral substituents on the porphyrin, successful nitrene
transfer to alkenes was achieved. Enantiomeric excess ranging from 43 to 68% and
product yields greater than 70% were obtained. In these catalysis studies, the reactive
intermediate was a Mn(IV) complex.
formation of a charge-
Figure 1.17: Suggested mechanism of olefm epoxidation catalyzed by MnTPP,
1.3.3. Immobilization of Porphyrins
The ability of porphyrins to efficiently catalyze both the epoxidation of olefins
and the hydroxylation of alkanes unfortunately leaves the porphyrin and its
superstructure vulnerable as potential substrates. However, nature has developed
mechanisms to eliminate these unwanted complications. For example, an enzyme and
its cofactors may form metal-oxo complexes only when the substrate molecule is
confined within an enzymatic cavity. Also, the tertiary protein structure prevents the
active porphyrin catalyst from approaching other potentially oxidizable
metalloporphyrins, and it makes the structure rigid, protecting the amino acid
backbone and the side-chains from intermolecular oxidation by contacting the active
site. These biosystems are difficult to mimic in the laboratory; however, successful
biomimetic catalysts have been prepared with bulky, rigid groups substituted on the
One alternative solution to the problem of internal oxidation or intermolecular
oxidative destruction of the porphyrin catalyst is immobilization of the chromophore.
Immobilization involves tethering the porphyrin to a surface such as a film,88 an
inorganic solid particle, '03-106 a polymer, '07,1 08 a membrane, '09 or a resin. "O
Immobilized porphyrins as biomimetics and as heterogeneous catalysts have been well
explored in the past several years. ' 04- 1 06, i lo Tethering of porphyrins to a solid support
can not only reduce or eliminate oxidative destruction of the active catalyst, but can
also aid in the catalyst recovery after the reaction is completed.
Heterocyclic ligands are commonly used as the link between the porphyrin and
surface in many immobilized porphyrin systems.94,98,11 1 Unfortunately, binding the
metallo-porphyrin to the imidazole allows little control over the porphyrin orientation
in the films. Additionally, in these circumstances, there is no chemical connection
between the porphyrin and the surface other than the ligand, which leaves the
porphyrin vulnerable to removal from the surface by ligand displacement, changing
the reaction conditions.30,94,98,1 1 1 ^n alternative method for tethering the porphyrins
to surfaces has been established, which uses four alkyl phosphonic acid substituents
that can be attached to a zirconium phosphonate network making a very stable
1.3.4 Heterocyclic Ligand Cocatalvsts
126.96.36.199. Li gand activation of the porphyrin catalvst . Heterocyclic ligands are
well documented in the literature as activating Fe(III) and Mn(III) porphyrins for
catalysis with oxidants such as alkyl or hydrogen peroxides.94,98,99 Porphyrins
immobilized on an ion-exchange resin support showed significant increases in
catalytic activity in the presence of either imidazole or 4-methylimidazole. With the
heterocyclic ligand present, nearly quantitative conversion of cyclooctene to
cyclooctene oxide was achieved, relative to only 5% conversion in the absence of
imidazole over the same time period. "2 Likewdse, Arasasingham et al. found a 4 to
10 fold increase in the rate of the reaction between a manganese porphyrin and an
oxygen source commonly used in olefin epoxidation reactions, t-BuOOH, in the
presence of imidazole. Since the oxidation of the porphyrin accelerates, a rate increase
should also be observed in the overall epoxidation reaction.98
According to Yuan and Bruice, the reaction of the Mn(III)TPP CI complexes
with peroxide oxidants only proceeds in the presence of a heterocyclic nitrogen base
ligand such as imidazole or pyridine. The imidazole ligation was pH dependent and
was evident only above pH 5. Consequently, the enhanced oxidation rate was also pH
dependent. Further, with common oxidants, nitrogen base ligation led to a significant
increase in the oxygen transfer rate. ^ ' •
The rate increase could be due to a general-base catalysis and/or ligation of the
imidazole (ImH) to the manganese ion.^s Activation by ligation of ImH is supported
by the fact that when 2,4,6-trimethyl-pyridine is used as the base, which is sterically
forbidden from porphyrin ligation, no increase in the reaction rate was observed.
However, when the ImH concentration was below a saturation level, the rate increase
was linear with ImH concentration up to a saturation level. An increase in the
oxidation rate with a basic ligand is likely due to the increase in the electron density at
the metal center arising from donation of the lone pair of electrons from the ImH.^s
The presence of the ImH as an axial ligand has been shown also to stabilize the metal-
0X0 compound. ^9
1 ■3.4.2. Spe ctral evidence of axial lip and. ImH to porphyrin binding should be
apparent in the UV-vis spectra. The formation of a bis-imidazole Mn(III)TPP CI
complex was demonstrated by a broadening shift in the Soret Band from 478 nm for
the pure porphyrin to 472 nm. ' 1 1 Interestingly, the equilibrium constants for the
formation of the mono- and bis-ligated imidazole-porphyrin complexes are similar and
their absorption spectra are nearly identical. Therefore, at high concentrations of
imidazole in solution, it is possible that the observed spectra arise from the formation
of bis-imidazole complexes." !'• '^ However, the preferred formation of the mono- vs.
bis-imidazole complexes has caused some disagreement, and some authors claim that
even at saturated concentrations of imidazole, the principal component is the mono-
In the UV-vis of the Mn-porphyrin, the Soret Band of the Mn-porphyrins is the
most sensitive to the axial ligand. The change in the Soret energy is due to the charge
induced on the porphyrin chromophore through the metal. Electron-donating axial
ligands induce negative charge on the macrocycle, separating the bonding and anti-
bonding orbitals of the porphyrin, and increasing the transition energy.! 15 Hard
anions, whose binding is strengthened by increased ionic character of the central
metal, prefer localization of positive charge on the metal, which leaves the
chromophore with more negative charge.' 15 Similarly, as a basic ligand takes on more
hard base character, the A.^3^ will shift to higher energies.
1 .4 Dissertation Overview
The overall goal of this dissertation was to prepare zirconium phosphonate thin
films by both the SA and LB technique that contained catalytic Mn-porphyrins. The
purposes of the zirconium phosphonate network were to stablize the manganese
\> '"-■■^■f 39
containing films to reaction conditions and to allow these films to be recycled in a
number of catalytic studies. Chapter 2 is an overview of the experimental techniques
used to prepare and characterize the films described in this dissertation, and materials
and instrumentation used in this pursuit are also presented.
Films containing a Pd-tetraphenyl porphyrin were prepared to develop fihn
preparation procedures and to better analyze the UV-vis properties of porphyrin
containing films. Substituted tetraphenyl porphyrins, palladium 5,10,15,20-
and palladium 5,10,15-tris(2,6-dichlorophenyl)-20- (2,3,5,6-tetrafluorophenyl-4-
octadecyloxyphosphonic acid)porphyrin (PdPl), have been studied as Langmuir
monolayers and as zirconium phosphonate LB and SA films.
Films were prepared incorporating the pure porphyrins and the porphyrins
mixed with octadecylphosphonic acid (ODPA). The Langmuir monolayers were
characterized with pressure vs. area isotherms and reflectance UV-vis spectroscopy.
Using a three-step deposition technique, symmetric and alternating zirconium
phosphonate bilayers and multilayers were prepared by the LB technique. PdP4
containing films were also prepared by the SA technique. In all PdPl and PdP4 films,
the porphyrin constituent resided in the hydrophobic region of the monolayer and the
phosphonate substituents bound zirconium ions in the hydrophilic region.
LB and SA films were studied with transmittance UV-vis and the LB films
were fiirther investigated using X-ray diffraction. Control over chromophore
interaction was achieved by chemical modification of the amphiphiles and by selection
of appropriate transfer conditions. For example, reduced aggregation was seen in LB
films of the tetraphosphonic acid substituted porphyrin PdP4 transferred at mean
molecular areas (MMA) larger than the area per molecule of the substituted porphyrin
and m SA films. In these films, the porphyrin macrocycles are non-aggregated and
oriented parallel to the surface. In contrast, the monophosphonic acid substituted
PdPl aggregates under all of the deposition conditions studied.
Stability of the Pd-porphyrin LB and SA films was examined by exposing the
films to refluxing chloroform. UV-vis absorbance after immersion in chloroform
confirmed conclusions that in films of PdPl, many of the chromophores are not
tethered to the inorganic network and are easily removed, whereas in films of PdP4, all
molecules bind to the zirconium phosphonate extended network making these films
Study of the Pd-porphyrins led to significant understanding of the behavior of
tetra- and mono-phosphonic acid porphyrins in LB films. Chapter 3 describes the
results of these studies, which were the first to show incorporation of porphyrins at the
exterior of metal-lattice containing films.
Manganese tetraphenyl porphyrins are well known epoxidation
catalysts,^'''99'"6 and the incorporation of these catalysts into zirconium phosphonate
films should improve their catalytic efficiency as well as their stability and
recoverability. Films containing manganese 5,10,15,20-tetrakis(2,3,5,6-
tetrafluorophenyl-4-octadecyloxyphosphonic acid)porphyrin (MnP4) have been
prepared using the LB and SA techniques. The formation of these films involved
modifying traditional LB procedures with SA techniques, which is possible with the
use of zirconium phosphonate networks. From Langmuir monolayer and LB studies
of the pure tetraphosphonic acid porphyrin, it appears that the MnP4 amphiphiles tend
to form face-to-face aggregates, or H-aggregates, when assembled at the air-water
interface, and this aggregation is translated into the transferred films. Attenuated total
reflectance (ATR) IR, UV-vis, XPS and stability studies confirm the presence of the
porphyrin. Thorough characterization of the MnP4 containing films is described in
The heterocyclic imidazole ligand has been shown to improve the catalytic
efficiency of Mn-porphyrins, and MnP4 films containing the imidazole ligand have
been successfully developed. These films were prepared by a variety of methods
involving a combination of LB, SA and substitution procedures. In solution, it is seen
that binding of a non-amphiphilic imidazole causes a small blue shift of the Mn-
porphyrin Soret band; however, a dominant influence on the Soret band in the films
and in solutions containing the ImODPA ligand comes from the metals axial
environment ~ especially halide binding. Mixed films containing both the imidazole
phosphonic acid (ImODPA) and the MnP4 molecules have been prepared and
characterized by ATR-IR, UV-vis, and XPS. The preparation and characterization of
imidazole and MnP4 containing films is presented in Chapter 5.
The epoxidation of cyclooctene using iodosylbenzene was catalyzed by the
pure MnP4 containing films with substrate to oxidant ratios of 20:5, 40:5, and 60:5
over a variety of reaction times. The self-assembled MnP4 films proved to have
slightly improved catalytic efficiency relative to the analogous LB films likely due to
the increased aggregation observed in LB deposited films. The mixed
ImODPA/MnP4 films showed catalytic activity in the presence of the peroxide
oxidants. These films were examined with different substrate to oxidant ratios. The
porphyrin containing films, both with and without ImODPA were resistant to
degradation under most examined reaction conditions. The catalysis results involving
both PhIO and H2O2 oxidants with pure porphyrin and mixed porphyrin/imidazole
films are described in Chapter 6.
2.1 Langmuir-Blodgett and Self-Assembled Films
2.1.1. General Langmuir-Blodgett and Self- Assembly Procedures
188.8.131.52. Fil m Formation. The general procedure for forming LB films starts
with the Langmuir monolayer, which are prepared on a Langmuir trough. The trough
consists of a rectangular piece of Teflon, typically 1 cm deep, supported on a metal
base with Teflon barriers, shown as black rectangles in Figure 2.1. A Teflon well is
carved in the center of a double barrier trough for transferring monolayers. The
spreading solution is prepared by dissolving the amphiphile of interest in a volatile
solvent, such as CHCI3. The solution is carefully applied to the subphase surface,
ideally spreading the molecules uniformly over the surface.
Figure 2.1: Schematic of Langmuir-Blodgett trough and monolayer.
' -■: :^ 43
The amphiphiles are shown in Figure 2.1 as gray circles, representing the
hydrophilic head group, and black lines, representing the hydrophobic tails. The
subphase, which is usually aqueous, must be nanopure. Because there is such a small
amount of amphiphile present, the monolayer is extremely sensitive to contaminants -
especially lipids and other surfactants and ions found in soaps and tap water.
The barriers compress the amphiphiles at a constant speed. In studying the U-
A isotherm, the film is compressed until it collapses. For LB transfers, the film is
compressed until the desired transfer pressure is achieved. At this point, the
monolayer is held at constant pressure for approximately two minutes until the
monolayer is stabilized, then the solid substrate is dipped vertically down through this
The monolayers are first characterized with TI-A isotherms. In modem
computer operated systems, the concentration (mg mL"') and the molecular weight (g
mol'), or the concentration in mol L"', of the compound being spread is entered into
the program along with the spreading surface area in mml From this information, the
program can calculate the MMA in A^ molecule "'. As the barriers move together and
the surface is compressed, the effective MMA is decreased and the surface pressure
The preparation of the zirconium phosphonate porphyrin films took place by a
three-step deposition procedure (Figure 2.2).28,29,38 a glass sample vial was placed in
the subphase in the well of the trough. Octadecylphosphonic acid was spread from 0.3
mg mL-' CHCI3 solutions and compressed at 15 - 20 mm min-' on the water surface.
At 20 mN m-', the substrate was dipped down through the monolayer surface and into
the sample vial at 8 mm min-i, transferring the ODPA template layer. The substrate
and the vial were then removed from the trough and an amount of zirconyl chloride
was added to the vial to make the solution ca. 4 x 10-5 M in Zr4+. After 20 min in the
zirconium solution, the substrate was removed from the vial and rinsed with water.
After the template layer was successfully prepared, it was dried and characterized
independently by ATR-IR, XPS, and UV-vis if needed.
Capping layers were prepared by a variety of methods, which will be described
for each different film type in Chapters 3, 4 and 5. LB or SA methods could be used
to form the capping layers. To form the capping layer and complete the zirconium
phosphonate bilayer by the LB technique, the now hydrophilic substrate was lowered
into the trough, a monolayer was spread on the surface and compressed to the desired
pressure, and the substrate was raised through the monolayer at 5 mm min'. To form
the capping layer by self-assembly, the hydrophilic surface was submerged in a
solution of the desired molecules at about 1 0"' M in an appropriate solvent, usually
EtOH/H20 (9/1). The capping layer was then allowed to self-assemble.
2.1.1 .2. Materials and Methods . Materials used to prepare the porphyrin
containing films included octadecylphosphonic acid (ODPA), zirconyl chloride
(ZnOCl-8H20), and the porphyrins themselves. The porphyrins were provided by
Bruno Bujoli, Fabrice Odobel, Karine LeClair, and Laurent Camus fi-om the
Laboratoire de Synthese Organique, at the Faculte des Sciences et des Techniques de
Nantes in Nantes, France. ODPA was used as purchased from Alfa Aesar (Ward Hill,
MA). Zirconyl chloride, 98% was used as supplied from Aldrich (Milwaukee, WI).
Octadecyltrichlorosilane (OTS) 95%, used to silanize and hence hydrophobicize the
substrates, was also used as purchased fi-om Aldrich. Amylene stabilized HPLC grade
CHCI3 was used as a spreading solvent, and was used as received from Acros
(Pittsburgh, PA) and Fisher Scientific (Pittsburgh, PA).
A KSV 2000 system (Stratford, CT) was used in combination with a
homemade, double barrier Teflon trough for the Langmuir monolayer studies and LB
from water surface
Sample vial in trough
- -; •
1 — »•«
•• — — ♦
♦ — — •
0» — — •
«• — — •
• * •
III r/::= =:•: .^ I II
Figure 2.2: Schematic of the three-step deposition process used for zirconium
The surface area of the 2000 trough was 343 cm^ (36.5 cm x 9.4 cm). A
platinum or filter paper Wilhelmy plate, suspended from a KSV microbalance,
measured the surface pressure. Subphases were usually pure water with a resistivity of
17-18 MQ cm-i produced from a Bamstead NANOpure (Boston, MA) purification
The films were transferred from the aqueous surface onto solid supports. Glass
microscope slides and glass coverslips were purchased from Fischer (Pittsburgh, PA)
and were used as substrates for UV-vis and catalysis studies. Single crystal silicon
wafers (10 0) were purchased from Semiconductor Processing Company (Boston,
MA), and cut using a diamond glass cutter to 25 mm x 15 mm x 0.8 mm for XPS
studies. These substrates were cleaned using piranha etch, which is 1 :4 H2SO4: 30%
H2O2, a new hydrophilic surface was prepared by the RCA procedure, • 17 which
involved first, heating in a 5:1:1 solution of water, 30% H2O2, and NH4OH, and
second, heating in a 6: 1 : 1 solution of water, 30% H2O2 and HCl. Then the substrates
were sonicated for 15 minutes each in methanol, 50/50 by volume methanol/
chloroform, and chloroform. The substrates were then sonicated in a 2%
octadecyltrichlorosilane (OTS) solution in hexadecane and CHCI3 for two hours.
Finally, the substrates were sonicated for 15 minutes each in CHCI3, 50/50 by volume
CH3OH/ CHCI3, and CHjOH.'is
184.108.40.206. UV-vis. Transmittance UV-visible experiments were performed on a
Cary 50 spectrophotometer by Varian with an average resolution of 2 nm. Porphyrin
solutions were studied by UV-vis in EtOH, H2O, CHCI3, and CH2CI2 solvents. The
behavior of the porphyrin with different potential ligands was investigated by mixing
the porphyrin solution with ethylphosphonic acid, t-butyl ammonium halides (chloride
and bromide) (Aldrich), and imidazole with no alkyl substituents (ImH) (Kodak). A 1
cm X 1 cm X 3 cm quartz cuvette held the sample, and the background using the
corresponding pure solvent was subtracted.
A Teflon substrate holder with grooves cut at 45° to one another was used to
obtain sampling at 0° (beam normal to the substrate) and 45° incidence to the substrate
surface. A plane, visible polarizer was used to select s- and p-polarized light.
Reflectance UV-vis experiments were performed on a KSV 2000 mini-trough using an
Oriel spectrophotometer and a 77410 filter with a range from 200 - 600 nm.
220.127.116.11. X-ray Photoelectron Spectroscopv. X-ray photoelectron spectroscopy
(XPS) was performed on a Perkin-Elmer PHI 5000 Series spectrometer using the Mg
Ka line source at 1253.6 eV. The instrumental resolution was 2.0 eV, with anode
voltage and power settings of 15 kV and 300 W, respectively. The operating pressure
was around 5x10'' atm. Survey scans were performed at a 45° takeoff angle with a
pass energy of 89.45 eV. During multiplex scans, 80-100 scans were run at each
peak over a 20-40 eV range with a pass energy of 37.35 eV.
2.1 .2.3 X-rav Diffraction . In order to obtain low angle X-ray diffraction
(XRD) patterns, multilayer films were transferred onto a hydrophobic glass slide. The
diffraction patterns were obtained using a Phillips APD 3720 X-ray powder
diffractometer with the CuKa line, A, = 1.54 A, as the source for films ranging from 10
to 15 bilayers.
18.104.22.168. Attenuated Total Reflectance Infrared . Attenuated total reflectance
infrared spectroscopy was performed on a Mattson Instruments (Madison, WI)
Research Series- 1 FTIR spectrometer equipped with a deuterated triglyceride sulfide
detector and a Harrick (Ossining, NY) TMP stage which held the Ge crystal substrate.
ATR-FTIR spectra consisted of 500 scans at 2 cm-' resolufion and were referenced to
the silanized crystal or previous bilayers.
': \( '
2.2 Porphyrin Films
2.2.1. Palladium Porphyrin Films
The palladium porphyrins studied were palladium 5,10,15,20-tetrakis(2,3,5,6-
tetrafluorophenyl-4-octadecyloxyphosphonic acid)porphyrin (PdP4) and palladium
5, 1 0, 1 5-tris(2,6-dichlorophenyl)-20- (2,3 ,5,6-tetrafluorophenyl-4-
octadecyloxyphosphonic acid)porphyrin (PdPl). The Bujoli group provided us with
these porphyrin amphiphiles.
The Pd-porphyrins made well-behaved monolayers; therefore, these
amphiphiles were often transferred by the LB technique. Additionally, the Pd-
porphyrins were studied in diluted mixtures with ODPA in an attempt to disrupt the
aggregate formation in the films. For mixed, Pd-porphyrin/ODPA films, the two
materials were simultaneously dissolved in a CHCI3 solution. The weighted average
concentration and molecular weight were calculated and used in the KSV software to
monitor the MMA with compression. Ratios of PdP (1 and 4) to ODPA studied
included 1:0, 1:1, 1:4, 1:9 and 0:1, respectively.
The creep of the pure palladium-porphyrin Langmuir monolayers was studied
at high and low pressures over 30 min, or slightly longer than the time of one
deposition. At a constant pressure of 12 - 15 mN m-', the area changed by 6% and
12% for PdP4 and PdPl, respectively. At low pressure (3 - 5 mN m-'), the change in
area was 3% and 7% for PdP4 and PdPl, respectively. The instability in the
monolayers led to a necessary correction in the transfer ratios. The corrected transfer
ratios for the pure PdP4 were 1 .0 - 1 .4 at high pressures and 1 .0 - 1 . 1 at low pressures.
For the pure PdPl , the corrected transfer ratios were 0.8 - 1 .0 and 0.9 - 1 .0 for high
pressure and low pressure transfers, respectively. The transfer ratios of the porphyrin
films mixed with ODPA consistently showed uncorrected transfer ratios near unity.
Monolayers of PdPl and PdP4 were also studied on both heated and basic
subphases. To heat the trough, an Isotemp Refrigerating Circulating Model 900
(Fisher Scientific) pump with a water/ethylene glycol bath was used. The temperature
of the subphase was monitored using a KSV thermosensor. A 0.01 M KOH solution
was added to the subphase to adjust the pH to the desired value.
PdP4 films were also studied by self-assembly. The SA solution was prepared
by diluting 1 mL of a 0.5 mg mL' solution of PdP4 in CHCI3 to 30 mL with a 9/1
EtOH/H20 mixture. The film was allowed to SA for approximately 2 hours before
studying by UV-vis.
2.2.2. Manganese Porphyrin Films
octadecyloxyphosphonic acid)porphyrin (MnP4) and a model porphyrin, manganese
5,10,15,20-tetrakis (pentafluorophenyl)porphyrin (MnPO) were prepared by the Bujoli
group. Again, ODPA was used for the template layers, zirconyl chloride was used to
prepare the zirconium network, and CHCI3 was used as the spreading solvent. Also, /-
butylammonium chloride (/-BuNHi^ Cr) (Aldrich) was used as a chloride source for
SA deposited films. NaCl (Acros) was used as the chloride source for LB transferred
To form the MnP4 monolayers, a 0.4 mg mL' solution was prepared in CHCI3
(often, in order to dissolve the porphyrins, up to 5% ethanol was added and the
solution was sonicated for about an hour). An appropriate volume of solution was
spread on the aqueous surface in order to reach and hold the desired transfer pressure
throughout the deposition. A variety of surface pressures were used for transfer and
will be described in more detail in Chapter 4.
To prepare SA Mn-porphyrin films, 1 mL of a 0.5 mg mL"' MnP4 solution in
EtOH was diluted to 30 mL with a 9/1 EtOH/HjO mixture in a 50 mL vial.
Alternatively, a 0.5 mg mL"' MnP4 solution in CHCI3 was diluted with pure CHCI3 or
CHjClj. The zirconated ODPA surface was exposed to the SA solution for 2 hr unless
otherwise specified. After 2 hr, there were typically some physisorbed chromophores,
which were rinsed off the surface using a hot solvent such as CHCI3 or CH3CN.
As is discussed in Chapter 4, halogenated solvents were originally chosen as
self-assembly solvents to eliminate possible ethoxide or water binding. However, the
oxide coating on the exposed zirconated ODPA template is soluble in EtOH/HjO
solvents making the zirconium available for binding the phosphonic acids of the
porphyrin capping layer. From UV-vis studies, the films formed fi-om these different
solvent systems appeared to behave similarly. Therefore, because the film formation
mechanism fi-om EtOH/H20 was better understood, this solvent mixture was normally
To induce chloride binding at the Mn-porphyrin' s axial position, a SA solution
containing approximately 0.01 - 0.1 M /-BuNH3'' CI" along with the porphyrin in
EtOH/HjO was prepared. The zirconated ODPA template was submerged in this
solution for 2 hr. Alternatively, chloride ions were incorporated into the aqueous
subphase used for LB transfer of the MnP4 monolayers using NaCl at 0.1 M and
From UV-vis results of the MnP4 and MnPO with ethylphosphonic acid, it
appeared that the phosphonic acid might cause the Mn(III)-porphyrin to go through a
spin state crossover fi-om high-spin to low-spin Mn(III). In order to examine this
magnefic change, the Evan's NMR method was used. "9. 120 a solufion of 10% /-
butanol in CDCI3 was injected into a small capillary tube using a long syringe needle.
The depth of the solution in the capillary tube reached approximately 2" . Two
standard NMR tubes were then filled to approximately 1" with sample solution. The
first was filled with pure MnPO (0.0106 g, 100 ^mol) dissolved in the 10% t-
butanol/CDClj solution. The second was filled with MnPO (0.0106 g, 100 ^imol) and
ethylphosphonic acid (0.220 g, 2000 ^mol). The reference solution in the capillary
was inserted into the porphyrin containing NMR sample, and the magnetic
susceptibility of the solute, Xg (cm^ g"') induced by the magnetic porphyrin was
where Af is the diamagnetic fi-equency shift, f is the spectrometer fi-equency, and m is
the mass of substance per mL of solution. Compared to literature values, the
differences in the Xg of MnPO with and without ethylphosphonic acid did not
correspond to a spin state change in the Mn(III).
2.2.3. Manganese Porphyrin/Imidazole Mixed Films
The general method used to prepare MnP4/imidazole films in this study
involved the initial formation of a zirconated octadecylphosphonic acid (ODPA)
template, as before. Onto this template, a film of either pure imidazole
octadecylphosphonic acid (ImODPA), which was prepared by the Bujoli group, or a
mixture of ImODPA and MnP4 could be formed. The zirconium phosphonate
network provided a means for locking the porphyrin and the imidazole into the films,
resulting in films that were stable toward the conditions used for the catalysis
reactions. Even under relatively harsh conditions such as elevated temperatures or
rapid solvent flow, the porphyrins films appeared to be stable. Additionally, the
zirconium phosphonate network made preparation of the MnP4-imidazole films
possible by a wide variety of mechanisms.
MnP4 and ImODPA could be incorporated into both LB and SA films. In
order to accommodate ImODPA into LB films, two types of spreading solutions were
used: 1) ImODPA mixed with a stabilizing agent such as hexadecylphosphonic acid
(HDPA), or 2) ImODPA mixed with the MnP4 amphiphile. Alone, the ImODPA did
not form sufficiently stable monolayers for transfer. The porphyrin, MnP4 was
substituted into the films of pure ImODPA or ImODPA/HDPA from an EtOH/HzO
solution. Alternatively, the MnP4 film was transferred by the LB technique and then
the ImODPA was substituted into these films. *,
First, in the case of the mixed 25% ImODP A/75% HDPA films, 200 ^iL of a
solution with a weighted average concentration of 0.17 mg/mL and a weighted average
molecular weight of 329.86 mg mmol"' was spread on the water surface and
compressed to 12 mN m"'. The film was transferred at 5 mm min"' on the upstroke
completing the zirconium network. Films made in this way were abbreviated
ODPA/Zr/25% ImODPA. These films were then placed in a solufion of the MnP4 at
ca.10-5 M in 9/1 EtOH/HjO and the porphyrin phosphonic acids were allowed to
substitute into the defect or vacant sites in the film for 2 hr. Transmission UV-vis of
these films confirms the ability to include the porphyrins by this method, and the
resulting films were called ODPA/Zr/25% ImODPA, SA MnP4.
Films containing both the MnP4 and the ImODPA transferred by the LB
technique from a mixed monolayer were also prepared. A 70/30 mixture of
MnP4/ImODPA, respectively, was dissolved in CHCI3 with a weighted concentration
and MW of 0.289 mg/mL and 992.70, respectively. 175 ^L of this solution was
spread and transferred at 3 mm min"' on the upstroke forming the ODPA/Zr/30:70
For the self-assembly of the imidazole onto a zirconated ODPA template, 2 mL
of a 0.5 mg mL' solution of ImODPA in EtOH was dissolved in a 9/1 EtOH/HzO
mixture in a 50 mL vial. The substrate containing the zirconated ODPA template was
placed in the vial and the film was allowed to self-assemble for 2 hr. When the self-
assembly procedure was complete, the film was rinsed with nanopure water and dried
with forced air. The MnP4 molecules were allowed to substitute into these films as
described in section 2.3.2. Similarly, mixed MnP4 and ImODPA solutions were
prepared at a variety of ratios in EtOH/HjO, and the mixed monolayer was allowed to
self-assemble for 2 hr.
The ImODPA was deprotonated as mentioned in Chapter 5 by soaking the
mixed film in an EtOH solution containing /-butyl amine. The /-butyl amine was used
to deprotonate the imidazole within the films so the ligand would be available for
binding the central manganese. The concentration of this solution was not precise but
was consistently ca. 0.1 M and rinsing times were ca. 15 min.
2.3.1 ■ Catalvsis using PhIO as an oxidant
Pure porphyrin films behaved as catalysts for the epoxidation of cyclooctene
using iodosylbenzene (PhIO) as the oxidant. PhIO was synthesized fi-om the diacetate
precursor using NaOH.i^i lodobenzene diacetate (3.0 g, 9.3 mmol) was placed in an
Erlenmeyer flask. 30 mL of 3 NNaOH was added with stirring over 5 min. The
mixture was stirred for 1 5 min and left to sit uncovered 45 min. 1 00 mL deionized
H2O was added with stirring and the yellow solid was filtered using a Buchner fiannel.
The solid was collected and washed with another 100 mL aliquot of H2O. The solid
was filtered again, and washed with CHCI3 2 times in a beaker and filtered. The solid
was dried in a vacuum desiccator. The product's melting point corresponded well to
the literature value of 210° C. lodometric titration, involving converting the PhIO
product to Phi and I^ with HI and titrating with sodium thiosulfate, gave a purity of
99%. 1 22
PhIO (27.5 mg, 125 ^mol) was diluted in 25 mL CH^Cl^. The PhIO compound
was only slightly soluble in CH^Cl^, so it was diluted and then sonicated for at least 30
min. After sonicating, an amount of the cyclooctene was added and the mixture was
stirred for about 1 min. Decane (97 ^L, 500 ^mol or 24 ^L, 125 ^imol), the internal
standard, was also added with stirring. ImL samples of this mixture were used for
both a blank run and a homogeneous run. In all homogeneous experiments, 1 ^iL of a
1 mM solution of MnPO was added to the blank solution to investigate the epoxide
yields with the porphyrin in solution.
Screws to hold
Figure 2.3: Schematic of catalysis cell, side view.
Approximately 5 mL of the above oxidant/substrate solution was transferred
into a small Erlenmeyer flask and from there loaded into the flow cells used for
studying catalysis with the films. Also, blanks and homogeneous solutions were
loaded into cells containing blank films (no catalyst) for studying product yields
affected by the flow cell.
Figure 2.4: Schematic of catalysis cell, top view.
The flow cells were built by the University of Florida machine shop. The cell
was made from two blocks of Delrin into which was carved a cell with dimensions:
0.96" X 1 .38" X 0.039" . Inlet and outlet tubes were place at the cell edges. One end
of a 1' length of 1/16" ID Viton tubing (Cole-Parmer, Vernon Hills, IL) was
connected to the inlet port and the other end was submerged in the reaction solution.
A Cole-Parmer Masterflex peristaltic pump (model 7553-70) (6-600 rpm) with an
easy-load head was used to introduce the solution into the cell, then the open end of
the Viton tubing was removed from the solution and connected to the outlet port of the
cell. The solution was circulated around the film using the peristaltic pump.
The reaction products were studied by GC. The GC instrument was a
Shimadzu GC-17A (Columbia, MD) with a hydrogen flame ionization detector. A 1
HL portion of the reaction solution was injected onto the 25 m, 0.025 mm ID RTX-5
column (Crossbond, 5% diphenyl-95% dimethyl polysiloxane). The column was held
at 50° C for 3 min and then ramped at 10° C min' for 15 min.
Sensitivity factors (k) were determined using decane and o-dichlorobenzene as
the internal standards. A series of runs were performed for both the cyclooctene
(CyO) and the cyclooctene oxide (CyOO) standards. Equation 2.2 was used to
calculate 'k' from the GC trace areas of the standard (AJ and the product (Acyoo) and
the known sample weights (Wcyoo and wj.
I^CyOO = (2.2)
After an average k value was obtained for the CyO and CyOO, the catalysis yields
were determined from the reaction mixture using Equation 2.3:
^CyOO = (2.3)
Because the PhIO oxidant was rather insoluble, a series of 1 mL aliquots of a
1.1 g L-' solution of PhIO in CH^Cl^ were dried and weighed. The final weights were
1.1 mg ± 9%, assuring us that the amount of oxidant in each reaction was
approximately the same.
In order to compare nearly the same concentrations of catalyst in both the
homogeneous and heterogeneous cases, the concentration of porphyrins in the films
was calculated to be, at most, 1 nmol. In was, however, difficult to keep this
concentration constant. In the homogeneous reactions, 1 nmol of the corresponding
non-amphiphilic MnPO was used.
Imidazole is reported to improve the catalytic efficiency of the Mn-porphyrins
in the presence of a peroxide oxidant, but for comparison, we also tried to see if
imidazole would improve the catalytic efficiency in the presence of PhlO. For this
experiment, 0.1 |a,mol of ImH was added to the blank and homogeneous solutions
using a PhIO solution with 40 |amol CO, 5 i^mol PhIO and 20 )Limol decane. Also,
films prepared by SA ImODPA and SA MnP4 were used in the flow cells with this
same PhIO solution. The reactions were, again, stirred for 24 hr.
2.3.2 Catalysis using peroxide oxidants
In studying the catalysis of the epoxidation of cyclooctene (CyO) with H2O2,
many different substrate to oxidant ratios were studied. The epoxide yields were
greatest, however, when very dilute solutions of reactants were used to keep the
proportion of reactants to catalyst near a factor of 10 and the substrate was used in
excess. The starting materials, CyO and HjOj, were dissolved in 250 mL of CH2CI2
with o-dichlorobenzene as the internal standard. ImL aliquots of this solution were
used for the reaction blank and homogeneous reactions and contained 8 |amol CyO,
0.2 )Limol H2O2, and 0.2 ^mol of o-dichlorobenzene. To the homogeneous reaction
were added 0.4 i^mol ImH and 0.001 |amol of MnPO. The original solution was
pumped into the flow cells using the peristaltic pump, and these reactions were
allowed to stir for 24 hr at room temperature.
PALLADIUM PORPHYRIN CONTAINING ZIRCONIUM PHOSPHONATE
3.1. Background on Palladium Porphyrin Films
Langmuir monolayers and LB films of the derivatized palladium tetraphenyl
porphyrin molecules, palladium 5, 1 0, 1 5,20-tetrakis(2,3,5,6-tetrafluorophenyl-4-
octadecyloxyphosphonic acid)porphyrin and palladium 5,10,15-tris(2,6-
acid)porphyrin, referred to as PdP4 and PdPl, respectively, are described in this
chapter. The central Pd metal is a four-coordinate, diamagnetic d* metal, and is co-
planar with the porphyrin ligand. Being four-coordinate, there are no complicating
axial ligands to consider.
Porphyrins PdP4 and PdPl are substituted with four and one
octadecylphosphonic acid groups, respectively. These molecules differ from many
other porphyrin amphiphiles in that there is a hydrophilic group at the end of the alkyl
chain substituent. In many literature reports, the porphyrin group is often the
hydrophilic part ofLB film forming amphiphiles.9' Molecules PdP4 and PdPl
(Figure 3.1) were designed to investigate whether porphyrins can be incorporated into
metal phosphonate LB films. Also, because of their well-understood spectroscopic
behavior, the PdP molecules were used to study how the orientation and aggregation
of the porphyrin can be controlled in the deposited films.
Figure 3.1: Structures of A) PdP4 and B) PdPl .
Palladium polyhalogenated porphyrins were chosen based on the following
considerations. First, palladium porphyrins are not demetallated in acidic conditions,
and their diamagnetic character makes following the synthesis with NMR
spectroscopy possible. Second, manganese and iron polyhalogenated porphyrins are
well-known catalysts for the oxidation of hydrocarbons; '23 therefore, if palladium is
replaced by one of these metals, the films can be used for catalytic applications.
Third, pentafluorophenyl substituents on porphyrins allow straightforward
functionalization of this ligand by aromatic nucleophilic displacement with an alcohol.
In addition, the ether bonds are more stable toward hydrolysis and less hydrophilic
than the ester or amide linkages usually used to tether alkyl chains on porphyrins.
High hydrophobicity of the linkage is a requirement because competition with the
polar phosphonic acid head-group should be avoided during the LB film preparation.
LB films of PdP4 and PdPl were formed incorporating a zirconium
phosphonate network. The strong tendency of zirconium ions to crosslink the
phosphonate groups precludes the normal deposition of organophosphonate
monolayers with the metal in the subphase.28 Therefore, a previously developed
three-step deposition procedure was used (Figure 2.2) as described in Chapter 2.28.38
Both symmetric (PdP/Zr/PdP) and alternating (ODPA/Zr/PdP) films have been
prepared in this way (Figure 3.2).
FT uTu S ii
= PdP1 and PdP4 I = OPA
Figure 3.2: Schematic of Pd-porphyrin films formed: a) alternating ODPA/Zr/PdP b)
alternating ODPA/Zr/PdP:ODPA mixed film, c) symmetric PdP/Zr/PdP, d) symmetric
Control over aggregation of the porphyrin chromophores is achieved through a
combination of molecular design and the carefiil choice of the conditions for transfer
of the films. Aggregation is decreased or eliminated in the films of the tetra-
substituted PdP4 when transferred at very high mean molecular area MMA and at high
subphase pH. Mixtures of this amphiphile with ODPA transferred at high
temperatures (40°C) and high MMA showed a similar decrease in inter-chromophore
interaction. The four long-chain phosphonic acid substituents significantly aid the
spreading of monomeric porphyrin species and the strength of the zirconium
phosphonate interaction assures their isolation in the transferred films. In similar
studies of the mono-substituted PdPl, aggregation was observed under all of the
transfer conditions explored, indicating that none of the deposition procedures
overcome the tendency of the molecules to aggregate.
3.2.1. UV-vis of Palladium Porphyrin Solutions
The palladium porphyrins show spectral responses in the UV-vis consistent
with hypso-type metallo-porphyrins. For each porphyrin, a strong Soret Band (or B
Band) is present above 400 nm and two Q Bands are centered around 550 nm.80.i24
Solution studies of the porphyrins PdP4 and PdPl were performed in ethanol and
chloroform, and the absorbance dependence on concentration was investigated.
Solutions ranging from IQ-i i M to 10-6 M were studied (Figure 3.3). In CHCI3, the
Soret Band was consistently at 410 to 41 1 nm for the porphyrin PdP4. In CHCI3,
therefore, PdP4 shows no sign of solution aggregation. For porphyrin PdPl at 10"' 1
M, the Soret Band absorbed at 41 1 nm; however, as the concentration was raised, the
Band shifted to 414 nm. Because PdPl has only one long chain substituent, the
likelihood of aggregation is increased; therefore, in CHCI3, the PdPl chromophores J-
aggregate at high concentrations.76,92 Interestingly, the Soret Bands for both PdP4
and PdPl absorb at 41 1 nm at lO'i i M, so the long chains have no effect on the Soret
Band of the non-aggregated chromophore.
Figure 3.3: Solution UV-vis of Pd-porphyrins in CHCI3: A) PdP4 B) PdPl The
absorbance scale refers to the lO"' M curve. The 10" M curve ha^ been enlarged for
Band comparison. ^
The studies of the same molecules at identical concentrations in EtOH and
water showed very different behavior (Figure 3.4). In EtOH at lO"' 1 M, both PdP4 and
PdPl show Soret Bands at 414 - 415 nm. This peak is significantly to the red of the
Soret Bands in CHCI3; however, it is known that more polar solvents tend to stabilize
the excited states in n-n* transitions, shifting this Band to lower energies.25.26 As the
concentrations of both PdP4 and PdPl were raised, the Soret Band systematically
shifted blue to 407 and 41 1 nm for the porphyrins PdP4 and PdPl, respectively,
implying H-aggregation of the chromophores in EtOH.76.92 Going from EtOH to
CHCI3 to H2O, at 10"* M, there is an obvious red shift in the X„^. This red shift does
not correspond to a solvent polarity shift, but it does correspond to a shift in the
solubility of PdP4. PdP4 is very soluble in EtOH and only slightly soluble in water.
' 1 '
'/ ^ \
li/ 1 \
J ' \
jf * \
// . \
/ / I
// \ \
._ 1 , 1 , 1 , r
350 400 450 500 550
Figure 3.4 Solution UV-vis of PdP4 in EtOH and water compared to CHCI3.
3 11 Langmuir Monolayers of Palladium Porphyrins
The room temperature pressure (D) vs. area (MMA) isotherm of PdP4 on water
at pH 5.5 is shown in Figure 3.5. There is a measurable onset of surface pressure near
220 A2 molecule- 1, followed by a gradual increase in pressure as the film is
compressed, with an apparent phase transition giving a steeper rise in pressure near
1 15 A2 molecule-'. The MM A of the tetraphenyl porphyrin is 200 A2 molecule-'
implying that at the onset, the tetrasubstituted porphyrin molecules are not aggregated
or stacked.9' However, this arrangement is not stable to pressure, and as the film is
compressed, the molecules are forced to rearrange.
50 100 150 200 250
Mean molecular area (A^ molecule ^)
Figure 3.5: Isotherms of PdP4, pure and mixed with ODPA (PdP4:0DPA), on a water
The change in the aggregation of PdP4 during compression can be observed
with reflectance UV-vis spectroscopy of the Langmuir monolayer (Figure 3.6). As the
film is compressed from a MMA of 370 A?- molecule' through 220 A2 molecule', the
>^max remains between 416 and 417 nm, similar to the X^ax observed for the non-
aggregated porphyrin in EtOH. At areas between 220 and 100 A2 molecule-', the Soret
Band shifts to 418 - 419 nm, and below 100 A2 molecule' the Soret Band shifts
fiirther to near 421 nm. The shift in the Soret Band suggests a change in the
interaction of the chromophores at different pressures. At MMA larger than and
comparable to the size of the chromophore itself, the porphyrin rings cannot be
aggregating to any significant extent or the onset of surface pressure would occur at
lower areas. The red shift of the Soret Band as the area is decreased indicates
enhanced chromophore aggregation at lower MMA.
380 400 420
Figure 3.6: Reflectance UV-vis of PdP4 on water subphase.
In contrast to PdP4, the U-A isotherm of PdPl (Figure 3.7) indicates that these
molecules aggregate even in the absence of applied pressure. No significant increase
in surface pressure is seen until areas below 120 A2 molecule-'. The pressure rises to
only 5 mN m-' at 60 A2 molecule-', below which the pressure increases until the film
collapses below 36 A2 molecule-'. The isotherm cannot reflect a true molecular
monolayer, but rather results from the compression of aggregates at the water surface.
Evidence of aggregation at all MMA is seen in the reflectance UV-vis spectra. Figure
3.8 shows the Soret Band as a function of MMA from greater than 120 A2 molecule-'
to film collapse at 36 A2 molecule-i. The Soret Band does not shift during
compression, and the X^ax of 426 nm indicates that the porphyrins are aggregated at
each stage of the isotherm.
20 40 60 80 100 120
Mean molecular area (A^ molecule"")
Figure 3.7: Isotherms of PdPl, pure and mixed with ODPA (PdPl :ODPA) on a water
A common procedure for enhancing the stability and processibility of unstable
Langmuir monolayers, and to reduce aggregation, is to mix the amphiphile of interest
with a good film-forming amphiphile.33,88-90 in this pursuit, both of the porphyrins
were mixed with ODPA, which is a well-studied amphiphile that forms a liquid-
condensed phase on the water surface and easily binds to an exposed Zr-phosphonate
surface. As the percentage of ODPA is increased, the isotherms increasingly take
characteristics of the liquid-condensed phase of ODPA, although features present in
the isotherms of the pure porphyrins are also present in the isotherms of the mixed
films (Figure 3.5 and 3.7).
1 ■ 1 ' 1 —
A/tU___ r .
. t 30
// 1 )\
//V\ft — ^
— 1 — i_i .
25 50 75 100 125 1£
MMA (A' molecule')
1 1 1 1 1
1 1 ,
360 380 400 420
Figure 3.8: Reflectance UV-vis of PdPl on water subphase.
In addition, the collapse pressure increases with the concentration of ODPA indicating
that the films become more stable as ODPA is added. However, diluting the porphyrin
film with ODPA does not appear to greatly affect the aggregation. Reflectance UV-
vis of a Langmuir monolayer of a 1 :9 mixture of PdP4 with ODPA is shown in Figure
3.9. The Xmax shifts from 415.5 nm at high MMA to 420 nm as the film is
compressed, just as it does in the films of pure PdP4 (Figure 3.9). However, the
porphyrins do not appear to be aggregated in the mixed film at high MMA.
Figure 3.9: Reflectance UV-vis of 10% PdP4: 90% ODPA on a water subphase.
The molecular areas in Figures 3.5 and 3.7 are weighted averages of the
porphyrin and ODPA molecules. The MMA of the porphyrin molecules in the mixed
films can be calculated using Equation 3. 1 : 89
where Smix is the MMA of the mixture determined from the isotherm, SpoR is the
MMA of the porphyrin within the mixed films, Sqdpa is the MMA of the ODPA
amphiphile in pure ODPA films, and N is the molar ratio of ODPA to porphyrin.
SpoR was calculated in the ODPA mixtures of each porphyrin at pressures of 5 mN
and 15 mN m-' and the results are plotted in Figure 3.10.
■ 5 mN m"^
2 4 6 8 10
— ■ 1 ■ 1 ■
■ 5 mN m'^
— 1 " 1 —
2 4 6 8 10
Figure 3.10: Mean molecular area vs. ratio of ODPA/Porphyrin: A) PdP4, B) PdPl.
If the ODPA diluent were breaking apart the preferred organization of the
porphyrins in the films, SpoR would increase as the aggregates separate. The decrease
in SpoR in the mixed films suggests that either the porphyrin chromophores are
reorienting in the mixed films or aggregation actually increases in the mixed films.
However, it does not appear that porphyrin aggregation decreases in the mixed
3.2.3 Langmuir-Blodgett Films
LB films of PdP4 and PdPl were prepared using the deposition procedure
described in Figure 2.2. The stepwise deposition allows fabrication of both symmetric
films, where the template and capping monolayers are the same, and alternating films,
where the two monolayers in the bilayer are different. Both types of films were
prepared for each porphyrin (Figure 3.2). It has been shown that a zirconated ODPA
template layer fi-equently provides the best substrate for transferring a capping layer.65
The extremely well organized and oxophilic surface allows deposition of almost any
phosphonic acid monolayer, including those that are not stable monolayers and would
normally not transfer. Monolayers of PdP4 and PdPl were transferred at a range of
temperatures, pressures and subphase pHs (Tables 1 and 2). Films of the porphyrins
mixed with ODPA were also transferred under a variety of conditions. Under some
conditions, perfect, organized monolayers were obviously not formed, but the films
could be transferred onto solid supports and studied.
22.214.171.124 Films of compound PdP4 . To form alternating films of PdP4, the
Langmuir monolayers were transferred as capping layers onto zirconated ODPA
template layers. Films were transferred at different surface pressures and the Soret
Band of the transferred films was used to monitor differences in chromophore
aggregation in the deposited films. The UV-vis spectrum of a film transferred at 130
A2 molecule-' (15 mN m-') is shown in Figure 3.1 1, where the A-max of the Soret Band
appears at 420 nm, significantly red-shifted from any of the solution spectra of PdP4.
The red-shift suggests increased aggregation, which is expected because at such a
small MMA, the chromophores must be either tilting perpendicular to the surface and
organizing side-by-side, or sliding over one another to form bilayers or multilayers.
Polarized UV-vis spectroscopy indicates the porphyrins are oriented parallel to the
surface, implying the latter arrangement.
Layers of PdP4 were also transferred at 190 (12mN m-') and
300 A2 molecule-' (Figure 3.1 1). The Soret Band shifts to 418 nm for the film
transferred at 190 A2 molecule-' and to 416 nm for the film transferred at 300 A2
molecule-', indicating less aggregation in films transferred at high MMA. At these
larger MMA, the porphyrin chromophores should be lying flat at the air-water
interface with little aggregation and they appear to remain non-interacting when
Table 3.1: UV-vis data from symmetric and alternating films of PdP4. X^^ is given
for monolayers, and interlayer thickness is given for multilayers of films transferred
under a variety of transfer conditions.
* Area of the chromophore and diluent as determined fi-om Figure 3.5 (isotherms)
** Corresponding pressure from Figure 3.5 (isotherm)
* pH of nano-pure water fi-om filtration system is about 5.5
1 ■-' 1 1 1 . 1 .—
r,\ 300 A' molecule-'
/ ' \
1 . 1 . t
400 450 500 550
Figure 3.11: Transmission UV-vis of PdP4 films transferred at high and low MMA.
Absorbance scale corresponds to the film transferred at 300 A^ molecule"'. The
absorbance for the film transferred at 130 A^ molecule' has been divided by 10.
As the pH was raised, the amphiphiles became slightly more water-soluble and
the monolayer was increasingly susceptible to creep. However, films of PdP4
compressed to 300 A^ molecule-i were deposited onto zirconated ODPA templates
fi-om subphases of pH 9.4 and 1 1 . 1 . As the pH increased, Pimax of the Soret Band of
the transferred film decreased to 414 nm for the film deposited at pH 1 1.1. This was
the lowest value of X-max, and therefore, the least aggregated LB transferred film of
PdP4. The A,niax of this Soret Band corresponds to that of chromophore PdP4 in EtOH
at 10-'2 M which is believed to be non-aggregated.
Consistently, D = 1 ± 0.02 when measured at 0° incidence, indicating no
preferred in-plane orientation of the chromophore in the PdP4 and PdPl films.
However, in all films, D ^ 1 when measured at 45 ° incidence. For films transferred at
high surface area, it is expected that the porphyrins should lie flat with all four
phosphonates tethered to the surface. Indeed, this is observed for films transferred at
190 A2 molecule-i and 300 A2 molecule-i where the tilt angle, 0, with respect to the
surface normal is observed to be 90°. Interestingly, the porphyrins also appear to lie
parallel to the surface in the films transferred at 130 A2 molecule-' where is also
measured as approximately 90°. This result implies that in films transferred at areas
smaller than the MMA of the flat porphyrin macrocycle, the molecules overlap,
stacking in bilayers or multilayers but with very little change in the tilt angle. There is
a larger uncertainty, possibly ± 10°, in the measurement as the tiU angles near 90'';26
however, these results confirm that the chromophores are lying approximately flat in
all of the films in this study.
Multilayers of the alternating 0DPA/Zr/PdP4 films can be deposited and X-ray
diffraction confirms the layered nature of the films. Two or three orders of the (00/)
Bragg peaks can be observed in each case. Films transferred at 190 A2 molecule"'
have a bilayer thickness of 42 A, which is smaller than the 48 A thickness seen in pure
ODPA/Zr/ODPA bilayers,28 suggesting that the 18-carbon tethers of PdP4 are not
fully extended in the alternating films. For the film transferred at 130 A2 molecule"',
the bilayer thickness increases to 47 A as the tetrasubstituted chromophores begin to
Symmetric bilayers of PdP4/Zr/PdP4 fabricated according to Figure 2.2, were
also studied. Porphyrin PdP4 could be transferred on the down stroke onto a
hydrophobic substrate under a variety of conditions. After zirconation, deposition of a
capping layer of PdP4 results in a symmetric bilayer. The Soret Band is very similar
to that from alternating films deposited at the same area per molecule, and polarized
UV-vis indicates the porphyrins are also lying parallel to the surface. However, the
layers are poorly organized, as (00/) Bragg peaks could not be seen in diffraction fi-om
9-bilayer films. It is probably poor organization in the template layer of porphyrin
PdP4 that is responsible for the lack of a well-defined layered structure.38
Mixed monolayers of PdP4 with ODPA were transferred onto ODPA templates
at different points along the surface pressure vs. area isotherms as shown in Table 1,
and the aggregation of the porphyrin in the transferred film parallels that seen in the
films of the pure porphyrins. For films transferred at pressures of 15 mN m-', the
Soret Band appeared at 420 nm, shifting to 415 nm when transferred at pressures less
than 5 mN m"' which, again, corresponds to the non-aggregated form seen in EtOH.
In all cases, polarized UV-vis indicates that the porphyrins orient parallel to the
The mixed monolayers show an interesting effect with increased temperature.
A mixed monolayer of 10% PdP4 with ODPA transferred at 15 mN m-' on a subphase
heated to 40° C shows a Soret Band X^ax of 415 nm, shifted fi-om 420 nm for the same
film deposited at room temperature. As the subphase is heated, the aggregates appear
to break-up in the mixed film. A similar effect is not seen on the pure films of PdP4.
It appears that the ODPA plays a role in breaking up the aggregated domains at higher
Films of PdP4 were also prepared by the SA technique. After the zirconated
ODPA template had been exposed to a PdP4 solution in EtOH/HjO for 2 hr, the
porphyrins were successftilly incorporated into these films. The Soret Band appeared
at 414 nm, which then shifted to 41 1 nm after 60 min rinsing in hot CHCI3. The X„^
in the SA film was the closest of any of the PdP films to that seen in the dilute
solution. Therefore, it appears that non-aggregated assemblies of PdP4 are easily
obtained by self-assembly (Figure 3.12). However, the overall absorbance intensity of
these non-aggregated films is lower than observed in the films transferred by the LB
technique at high MMA.
-0.005' ' L
rinse 60 min
-J 1 u
350 375 400 425 450 475
Figure 3.12: UV-vis of SA PdP4 films rinsed in hot CHCL.
126.96.36.199. Films of compound PdP 1 . The ll-A isotherms and the reflectance
UV-vis experiments described above indicate that the molecules of PdP 1 aggregate
upon spreading, and this aggregation is preserved in the transferred films. In contrast
to PdP4, the monophosphonate PdPl is only slightly influenced by attempts to break
up the aggregates by changing the deposition conditions. The UV-vis spectrum of a
capping layer of PdP 1 transferred at 52 A^ molecule-' (12 mN m-i) is shovm in Figure
3.13, where the Soret Band appears at 426 nm, consistent with the value observed in
the reflectance spectrum taken fi-om the water interface. The shape of the Soret Band
does not change for films deposited at higher MMA, higher temperatures, or in
mixtures with ODPA. The peak position shifts only slightly (Table 2). The
orientation of the chromophores were also unaffected by the deposition conditions.
Polarized spectra consistently give tilt angles of 90°, corresponding to the porphyrins
Figure 3.13: Transmission UV-vis of films of PdPl transferred at high and low
X-ray diffraction from alternating films of PdPl transferred at 52 A^
molecule-' onto a zirconated ODPA template gives a layer thickness of 61 A (Table
3.2). This thickness is larger than that of the alternating films of 0DPA/Zr/PdP4 or
ODPA/Zr/ODPA bilayers.28 Since optical spectroscopy indicates the molecules lie
flat, the enhanced thickness of the layer suggests they transfer as stacked bilayers or
multilayers. Further evidence for this arrangement comes fi-om the film stability
studies, described below, which indicate that part of the transferred film of porphyrin
PdPl is physisorbed to the surface.
Table 3.2: UV-vis data from symmetric and alternating films of PdPl. X„^ is given
for monolayers, and interlayer thickness is given for multilayers of films transferred
under a variety of transfer conditions.
10% PdPl /Zr/
* Area of the chromophore and diluent as determined from Figure 3.7 (isotherms)
** Corresponding pressure from Figure 3.7 (isotherm)
*** pH of nano-pure water from filtration system is about 5.5
188.8.131.52. Film stability . Zirconium phosphonate LB films are insoluble in
organic solvents due to the cross-linking within the zirconium-phosphonate extended
network.28 In order to monitor how well the porphyrin layers bind to the zirconated
template, transferred films of PdP4 and PdPl were rinsed with chloroform in a Soxhlet
extractor. Figure 3.19 shows the Soret Band absorbances as a function of washing
time in hot CHCI3. Figure 3.14 shows that none of the film of PdP4 is washed away
after 1 hr in chloroform, suggesting that all of the molecules are tethered to the
zirconated ODPA template. The same result was obtained for films of PdP4 deposited
at both higher and lower pressures or in mixtures with ODPA.
In contrast, the absorbance of the LB films of PdPl exposed to CHCI3
decreased significantly due to the desorption of chromophores. The absorbance
leveled off after 20 min, to a value corresponding to the truly surface confined
chromophores (Figure 3.14). This result suggests that the stacked layers of
chromophores in the porphyrin PdPl films were partially physisorbed on the surface.
^— « — » D— Q -« —
— ■^*— ><
10 20 30 40 50 60
Time in Soxhlet (min)
Figure 3.14: Absorbance of Soret vs. time rinsed in hot CHCI3: a) PdP4, b) PdPl .
The behavior of the monosubstituted and tetrasubstituted porphyrins is very
different on the water surface, and the differences are carried over to the transferred
films. Porphyrin PdPl spreads on the water surface to a limited extent. Our proposal
for how the molecules behave on the water surface is shown schematically in Figure
3.15. Optical spectroscopy indicates that PdPl aggregates, but the ti-A isotherm and
X-ray diffraction from the transferred layers suggest that the aggregates are only a few
molecules thick. The aggregates are present at both high and low MMA and can be
transferred, as aggregates, onto the zirconated ODPA templates. Some molecules
from each aggregate chemisorb to the zirconated surface through zirconium
phosphonate linkages, but some are physisorbed as part of the preformed aggregates.
When exposed to hot chloroform, the physisorbed part of the film is dissolved away.
1 , 1 L^ -, I
I — ~i r—p
20 40 60 80 100 120 140
MMA (A' molecule')
Figure 3.15: Illustration of orientation and packing of PdPl films transferred at high
and low MMA.
Chemical modification with four alkylphosphonic acid sidegroups allows the
porphyrin to spread completely. Porphyrin PdP4 spreads to a monolayer thick film at
high MMA, and as the film is compressed, an increase in surface pressure is registered
near 200 A^ molecule-', corresponding to the area of the flat porphyrin macrocycle.
However, the side-by-side arrangement of the porphyrin chromophores is not stable as
the pressure is increased and the film rearranges, with the molecules sliding over one
another to form multiple chromophore layers. This behavior is illustrated in Figure
100 150 200 250
MMA (A^ molecule^)
Figure 3.16: Illustration of orientation and packing of PdP4 films transferred
at high and low MMA.
The films of PdP4 can be transferred at high MMA onto zirconated ODPA
templates to form monolayer or submonolayer films of the porphyrin chemisorbed to
the surface with the chromophore ring oriented parallel to the surface. Films of PdP4
can also be transferred at lower MMA, where the reflectance UV-vis indicates the
porphyrins are interacting. Analysis of the transferred films suggests that the
porphyrin chromophores are lying flat and overlapping each other to form layers that
are a few molecules thick. In contrast to the films of PdPl, all of the molecules in the
aggregated films of PdP4 appear to be chemisorbed to the surface. None of the film is
lost during rinsing with hot chloroform. The different behavior probably results from
the fact that four phosphonic acid groups increase the chance of each molecule
bonding onto the zirconium phosphonate network. Any orientation of the porphyrin
macrocycle will direct at least one alkylphosphonic acid side chain toward the surface.
Also, all of the phosphonic acids have the potential to reach the water surface at high
MMA. Forcing a strongly hydrophilic group off of the water surface requires more
energy than reorganization of the alkyl chains or shifting the chromophore
Whether the Langmuir monolayers are transferred intact or reorganized during
film transfer is not yet completely clear. At high MMA, molecules of PdP4 lie flat on
the water surface and this arrangement appears to be preserved in the transferred films
based on the similar X^^. However, when the films are transferred at lower MMA,
where the films are clearly aggregated, there could be some rearrangement. There is a
significant driving force for forming zirconium phosphonate linkages, and the
aggregates could rearrange during transfer to fiarther maximize interactions with the
zirconated surface. While the porphyrins are clearly oriented parallel to the surface in
the transferred film, providing each porphyrin the chance to form multiple zirconium
phosphonate bonds, it is not known if the molecules aggregate the same way on the
The LB procedure used in these studies takes advantage of the binding energy
of the zirconium phosphonate continuous network and is shown to be quite versatile.
Both symmetric and alternating layer films can be prepared. Use of the zirconated
ODPA template layer allows almost any phosphonic acid derivatized amphiphile to
transfer in a capping layer.28.38,65,i25 Unlike conventional LB depositions, films of
PdP4 and PdPl can be transferred onto the zirconated template layers at any surface
pressure, allowing, in the case of PdP4, the arrangement of the molecules in the
transferred films to be tuned by choice of the area-per-molecule at deposition. The
films do not need to be stable Langmuir monolayers in order to transfer, as the driving
force is formation of the zirconium phosphonate bonds. It is the strength of the
zirconium phosphonate interaction, in particular the lattice energy associated with the
zirconium phosphonate extended network, that is responsible for the exceptional
stability of these non-traditional LB films.28.38
Zirconium phosphonate LB films, like solid-state zirconium phosphonates, are
insoluble in organic solvents and under most aqueous conditions. The inorganic
network has also been shown to enhance the thermal stability of LB films. 1^6 The
zirconium phosphonate inorganic extended network adds substantial stability to the
films, which are insoluble under most organic and aqueous conditions. The methods
developed here with the palladium tetraphenyl porphyrins can also be applied to other
porphyrin systems, and in this way the vast array of physical and chemical
characteristics of porphyrins, including catalytic activity, should be able to be
incorporated in stable LB films.
MANGANESE PORPHYRIN CONTAINING
ZIRCONIUM PHOSPHONATE THIN FILMS
Monolayer and film work using the molecule manganese 5,10,15,20-
MnP4, will be discussed in Chapter 4 (Figure 4. 1 A). For comparison, work done
using a similar molecule without the four alkylphosphonic acid chains, manganese
5,10,15,20-tetrakis(penta-fluorophenyl)porphyrin, or MnPO, will also be discussed
(Figure 4. IB). The manganese porphyrins are structurally and chemically more
complex than the palladium porphyrins. The Mn(III) central metal is a 5- or 6-
coordinate d'* metal. 80, 127, 128 Depending on the ligand character, Mn(III) is either S =
2 (high spin) or S = 1 (low spin). 129 Also, depending on the axial ligand or ligands,
the Mn(III) may or may not be co-planar with the porphyrin ligand. Mn(III) also has
an easily accessible lower oxidation state, which leads to significant metal/porphyrin
electronic interactions.^o the Mn(lIl)-porphyrins have a tendency to form face-to-face
dimers bridged through an axial ligand, 130 and the Mn(III)-porphyrins are vulnerable
to demetallation under certain conditions. Therefore, film characterization using these
molecules was much more complicated than with the Pd-porphyrins.
To investigate the catalytic properties of manganese-porphyrin films, film
preparation procedures involving the tethering of Mn-porphyrins to a metal
phosphonate network were developed. This method involves the initial formation of a
zirconated octadecylphosphonic acid (ODPA) template onto which a film of pure
MnP4 can be SA or transferred via the LB technique (Figure 2.2). Including MnP4 in
a zirconium phosphonate network provided films that were stable toward harsh
organic conditions. Also, the strong oxophilicity of the zirconium for the phosphonate
oxygens enabled the film preparation procedure to be easily altered and fine tuned and
complete film characterization to be carried out.
Figure 4. 1 : Structures of A) MnP4 and B) MnPO.
The MnP4 molecule is similar to the PdP4 molecule described in Chapter 3, in
which the manganese tetraphenylporphyrin (MnTPP) chromophore and the strongly
hydrophilic phosphonate groups are separated by 18-carbon chains (Figure 4.1 A). This
geometry allowed for the porphyrin to be sitting at the exterior of the film and
available for catalysis while the phosphonates were buried in the hydrophilic region
and available for binding to the stabilizing inorganic network. The incorporation of
this network significantly improves the resistance of the film to typically destructive
forces such as solvent, heat, or time.'26 in addition, having four amphiphiUc chains on
the porphyrin permits the formation of Langmuir monolayers of these materials
without diluting the amphiphiles with a good film forming amphiphile such as stearic
The MnP4 films were first investigated on the water surface in a Langmuir
monolayer. An isotherm of this material showed significant film compressibility
(Figure 4.2). Reflectance UV-vis showed that the porphyrins formed face-to-face
dimers above ca. 10 mN m'', which were maintained upon transfer onto glass
The procedure for MnP4 film formation was directed by the film
characterization results. In most cases, evidence suggests that the phosphonic acid
tethers on the porphyrins were able to bind to the metal phosphonate lattice. Film
stability was monitored by UV-vis, which displayed no significant chromophore loss
after 5 minutes in hot CHCI3 or CHjClj. Although the zirconium phosphonate lattice
contributes no interesting physical phenomena to the final film, the strong oxophilicity
of the phosphonate oxygens for the zirconium lattice allow for a wide variety of stable
films to be formed.
4.2 UV-vis Behavior of MnTPPs
4.2.1. Solution Studies
Both MnTPPs displayed electronic behavior in solution consistent with d-type
hyperporphyrins. Mn(II)-porphyrins have absorption spectra similar to free-base
porphyrins due to a lack of metal-porphyrin interaction; however, the absorption
spectrum of the Mn(III)-porphyrin is quite different. According to Gouterman,^^
Mn(III)TPP is a classic d-type hyperporphyrin with extra absorption bands at higher
energies relative to A,n,ax- The MnP4 and MnPO with chloride axial ligands are
spectrally consistent with d" porphyrins in high-spin configurations. '^i The strong
absorption near 450-485 nm is often called Band V, due to the fact that this transition
is not pure n-n* in nature but includes metal-porphyrin orbital mixing. However,
traditionally, it is still often called the Soret Band.^o One strong band commonly seen
to the blue of the Soret Band is called Band VI. A prominent peak, often observed
specifically in the MnP4 UV-vis spectrum ca. 410 nm, is referred to as Band Va.
Ligand effects and orientation or aggregation effects are reflected primarily in
the shape or shift of the Soret Band and in the extinction coefficient (s).>32 Therefore,
the behavior of this band was carefully monitored. Also, different solvents used for
porphyrin investigations caused changes in the absorption spectra. If a coordinating
solvent was used, the axial ligand was displaced by a solvent molecule causing a shift
in the Soret Band and in the VA^I intensity ratio.80>'32
184.108.40.206. UV-vis of MnPO in solution . A concentration study of MnPO in
CH2CI2 showed that the >.max was consistently at 475 - 477 nm between 10-5 and 10-8
M (s = 1.25 X 10' M"' m"'). These results suggest that the MnPO chromophore had
little tendency to aggregate in these dilute solutions, and that the axial ligand was
probably chloride. '32 In EtOH, the X^ax of the MnPO solufion was also constant over
the same concentration range; however, the Soret Band was blue shifted to 454 - 456
nm. According to Mu, the coordination of two axial methanol ligands to a MnTPP
caused a 10 nm blue shift relative to the chloride bound moieties; therefore, it is
believed that this blue shift is due to bis-EtOH binding. '33 Figure 4.2 shows the
solution UV-vis of MnPO in EtOH and CHCl, at 10"' M. Not only is the Soret Band
shifted, but the ratio of Band V to Band VI has also changed, indicating a change in
the metallo-porphyrin ligand environment.
Figure 4.2: UV-visofMnPOinCHClj
4.2. 1.2. UV-vis of MnP4 in solution . Similar solvent shifts were observed in
the case of MnP4. In CH2CI2 or CHCI3 at 10'* M or lower concentrations, the Xmax
appeared at 463 - 464 nm, whereas in EtOH or water, the symmetric Soret Band
occurred at 454 nm (Figure 4.3). The peak at 454 nm corresponds to the MnPO Soret
Band in EtOH indicating a similar axial environment, likely a bis-ethanol complex;
however, a very clear band, Va, was present at 418 nm in the MnP4 solutions.
Additionally, the ratios of Band V to VI were different for each different solvent,
implying that coordinating solvents effect the porphyrin axial environment.
The observed MnP4 spectral behavior in CHCI3 was very different from the
spectral behavior in EtOH or HjO. At 10"* M, the X^^ occurs at 463 nm with a distinct
shoulder present on the red side of the Soret Band. As the concentration of porphyrin
in CHCI3 increased from lO-^ M to 10"^ M, the red shoulder became a distinct second
peak at 477 nm (Figure 4.4). The two peaks are hereafter referred to as Vi and Vii for
the first and second Soret Bands. The peak at 477 nm was identified as the formation
of a five-coordinate MnTPP CI structure in the case of MnPO in CHCI3, and therefore,
it clearly represented the analogous structure in MnP4 (Figure 4.2).
1 \ .CHC13
/ \ Va 7
/ 1 \
/ \ \
/ 1 \
/ \ \
■ ' T ■ 1 •
400 450 500 550
Figure 4.3 Solvent behavior of MnP4 in water, EtOH and CHCI3.
350 375 400 425 450 475 500 525
Figure 4.4: UV-vis concentration study of MnP4 in CHCI3: a) 10* M, b) 10"' M.
-yf — l.J*^"'
The obvious difference between MnPO and MnP4 is the presence of the four
C,8-chain phosphonic acid tethers linked to the phenyl rings of the MnP4. Therefore,
the peak at 464 nm is, as previously mentioned, probably due to intramolecular
phosphonate binding. However, the structures of the two chromophores also differ in
that MnPO has four pentafluoro-phenyl substituents, and MnP4 has an ether linkage to
the alkyl chain at the para-position of the phenyl groups. For a more direct
comparison, a compound incorporating similar /7-methoxy-tetrafluoro-phenyl
substituents was prepared (MnP-4MeO). This compound's spectral behavior was
identical to the MnPO compound, further indicating that phosphonate binding causes
the Soret peak at 464 nm.
4.2.1 .3. MnPO in solution with ethylphosphonic acid . Though the alkyl chains
may have altered the chromophore interaction in solution, '34 the stronger UV-vis
effects were likely due to the presence of the intramolecular R-P0(0H)2 ligands. The
binding of the phosphonic acids to the central porphyrin metal was confirmed by
studying solutions of MnPO (1 x 10"' M in CHCI3) with ethylphosphonic acid at high
concentrations. In pure MnPO, the X^^ was observed at 475 - 477 nm; however, when
1 X 10"* M ethylphosphonic acid was added, a distinct blue shoulder became visible.
At ethylphosphonic acid concentrations above 3 x 10" M (over 1000 times the
porphyrin concentration) the dominant peak was the peak at 460 nm (Figure 4.5).
Further, upon addition of phosphonic acid to MnPO solutions in CHCI3, an obvious
peak emerged at 41 8 nm. Band Va was absent in the pure MnPO solutions, hence, this
peak was probably related to phosphonic acid binding.
Figure 4.5: MnPO in CHCI3 (1 x 10"' M) with ethylphosphonic acid: a) pure MnPO, b)
1x10- M ethylphosphonic acid, c) 2 x 10" M ethylphosphonic acid, d) 3 x 10"" M
ethylphosphonic acid, e) pure MnP4.
The peak at 41 8 nm could represent many different states of the porphyrin. It
could be due to the oxidation of the central metal, to a spin state conversion, or even to
demetallation of the manganese porphyrin. This peak was observed in the absence of
any strong oxidants indicating that this peak is probably not representing oxidation
products. .\ , . •
The electronic spectra of Mn(III)TPP CI in DMSO was presented by Hansen
and Goff.129 The high spin moiety that was formed by the coordination of the DMSO
solvent molecules showed a sharp Soret Band at 465 nm with two prominent higher
energy bands at 375 and 397 nm. However, when the molecule was converted to a
low-spin Mn(III)-porphyrin complex by the bis-axial ligation of imidazolate anions,
the Soret Band shifted to 451 nm and broadened. Also, the band at 397 nm increased
in intensity. '29 In a similar way, the phosphonic acid or phosphonate may have been
behaving like the imidazolate anion and converting the MnPO from a high spin to a
low spin complex. The spin-state of MnPO in solution was studied using the Evan's
method (described in Chapter 2). The NMR results indicated that the MnPO with and
without phosphonic acid ligands were in the same spin-state, therefore, the observed
spectral behavior was due only to the ligand environment of the porphyrin.
The peak at 418 nm aligned with the Soret Band of the corresponding free-base
porphyrin in CHCI3 (g = 9.0 x 10^). The strong attraction between manganese and
phosphonates is well known; therefore, irreversible binding of the phosphonate to the
manganese may result in the demetallation of the porphyrin. The demetallation could
be facilitated by the protic nature of the phosphonic acid. This band was often
observed, but its relative intensity was not consistent.
When the MnPO/ethylphosphonic acid solution spectra were overlayed with a
solution spectrum of pure MnP4, the red shoulder, Band Vii of the MnP4 spectra,
aligned with the original MnPO peak. Also, the blue shoulder. Band Vi, which
emerged with the addition of the ethylphosphonic acid to a MnPO solution, was similar
in energy to the original MnP4 peak.
That such high concentrations of ethylphosphonic acid were necessary to
induce a change in the MnPO spectrum indicates that the five-coordinate MnTPP(Cl)
structure was generally favored over the six-coordinate MnTPP(Cl)(PA) structure (PA
= phosphonic acid). However, with the four phosphonic acids linked to the MnP4
chromophore through the alkyl chains, the effective concentration of phosphonic acid
in the vicinity of the metal was very high. Therefore, MnP4 in CHCI3 solutions favor
phosphonic acid axial ligation. Considering the Soret Band in the above MnP4
concentration study (Figure 4.4), it is apparent that this preference is stronger in the
more dilute solutions of MnP4.
220.127.116.11. MnP4 in solution with chloride ions . If the UV-vispeakat 460 nm
demonstrated the porphyrin's propensity to bind phosphonic acids, it was important to
determine if the phosphonic acid could be replaced with another ligand such as
imidazole (ImH), or chloride. According to Arasasingham, the displacement of a
water or hydroxy ion by imidazole is encouraged by the electron-donating nature of
the nitrogen base ligand. "4 Unfortunately, no literature precedence was found on the
strength of the phosphonate binding. Though chloride binding was implied in
concentration studies of MnP4 in CHCI3, it was further tested by adding tetra-
butylammonium chloride, /-BuNHj^ CI, to solutions of MnP4 at constant
concentrations of 1 0^' and 1 0"'' M (Figure 4.6). Similar behavior was observed with
added bromide ions, with Band Vii shifted to slightly lower energies. As the chloride
concentration increased, the red shoulder at 477 nm seen in the original MnP4 spectra
in CHCI3 became dominant and the blue shoulder, indicating phosphonic acid binding,
disappeared completely. . ? . s .
— ' 1 ' 1 • 1 ■
^^=^^ \/ ^^^
1 ' ..-,.- , 1 ,
425 450 475 500
Figure 4.6: Solution UV-vis investigation of MnP4's sensitivity to displacement of R-
PO(OH)' by chloride at 1 x 10' M (CHCI3). The arrows indicate the changes in the
intensity of the peaks as the chloride concentration changes from 0.0 M to 0.1 M while
the concentration of MnP4 was constant.
4.2.2. Langmuir Monolayers
The monolayer behavior of MnP4 was studied using surface pressure (IT) vs.
Area (MMA) isotherms on water (Figure 4.7). Investigating the tilt angle of the
chromophores transferred at various surface pressures by polarized UV-vis gave some
indication of the chromophore orientation in the monolayers. The fl-A isotherm
showed a distinct n onset at ca. 250 A^ molecule"'. However, the approximate area of
the chromophore itself is ca. 200 A2 molecule'. This large onset area implied that the
alkyl chains were initially buckled so both the chromophores and the phosphonic acid
groups were sitting on the water surface. The hydrophilic nature of the porphyrin,
especially if the two axial positions are coordinated with water, makes this a viable
scenario. In the first region of the isotherm ('a' of Figure 4.7), the chromophores and
the phosphonic acids remained on the water surface and were simply compressed.
50 100 150 200 250 300
Figure 4.7: Isotherm of MnP4 on water subphase.
During the plateau region of the isotherm ('b' of Figure 4.7), the chromophores were
pushed off the water surface and the porphyrin rings started overlapping. There were
likely some dimers forming and not all phosphonic acid end groups reached the
surface. In region 'c' of Figure 4.7, the MMA was approximately 100 A^ molecule"',
implying the presence of dimers, which were then further compressed until the
"monolayer" collapsed. At this point, the film was likely not a true monolayer as the
chromophores essentially formed a bilayer.
The separation of the chromophores from the water surface was accompanied
by overlapping, which was also indicated by a consequent shift in the Xmax detected by
reflectance UV-vis (Figure 4.8). In region 'a', the X^^ax was at 457 nm. While in
region 'b', the Soret Band shifted to and remained at 453 nm. The blue shift
corroborates the ft)miation of face-to-face dimers. The mean molecular area of this
spectral shift indicated that dimers began forming even before all of the chromophores
were pushed off of the water surface.
I ■-■ —
Jdj JfJMi Y ^nnA^mr^i '
420 440 460
Figure 4.8: Reflectance UV-vis of MnP4 on water subphase.
4.2.3. Langmuir-Blodgett Films of pure MnP4
18.104.22.168. Deposition of MnP4 from a pure water subphase. Using the LB
technique, the MnP4 monolayers were transferred at various points along the isotherm.
The transmittance UV-vis spectra of these films showed that the blue shift in the Xmax
followed the shift observed in the reflectance UV-vis experiments on the water surface
(Figure 4.9). Films transferred around 5 mN m"' had a X-max at 462 nm, while films
transferred at pressures higher than 10 mN m"' were blue shifted to 456 nm. The Soret
Band blue shift suggested that the chromophores were interacting as H-type
aggregates. The chromophore interactions observed on the water surface were similar
to those observed in the deposited films; however, the X^^ at ca. 460 nm also implied
that the chromophores were intramolecularly ligating phosphonic acids.
' 1 ' 1
1 1 1 1
1 15mN/m :a
1 1 I ■— ,
, . 1 1
350 400 450 500
Figure 4.9: UV-vis of MnP4 capping layers transferred onto ODPA/Zr at different
The polarized UV-vis spectra were studied to better understand the orientation
of the chromophores within the monolayer. In the case of a high- spin, 6-coordinate
Mn(III)TPP, the metal typically lies within the porphyrin macromolecular plane. '27
Therefore, the tilt angle of the chromophore with respect to the surface can be
determined from polarized UV-vis as described in Chapter 2.25,26,135 When the
monolayer was transferred in region 'a' of Figure 4.7, the tilt angle was determined to
be ca. 90°. Within the plateau region of the EI- A isotherm, the tilt angle immediately
after transfer was as low as 60°, but within minutes, the chromophore relaxed back to a
90° orientation relative to the surface normal. When transferred after the plateau
region, at areas much smaller than the chromophore itself, the tilt angles were
consistently ca. 90°, further confirming the presence of stacked rather than tilted
The stability of films transferred by the LB technique before, during, and after
the plateau region of the isotherm were tested by exposing the films to hot CHCI3 for
up to 60 min each. When transferred at 15 mN m'' (Figure 4.10A), the original Soret
Band was at 455 nm. After rinsing, the Band Vi shifted to 463 nm and a second Soret
Band appeared at 477 nm. This red shift in the band associated with phosphonate
binding is probably due to elimination of H-aggregated, physisorbed chromophores.
Band Vi observed before and after rinsing in the films transferred at high MMA,
remained at the same energy indicating that there was no significant change in the
chromophore interaction at this transfer area. Because chromophore interaction was
expected to be low at this MMA before rinsing, it was not surprising that no shift was
observed. In each case, the absorbance intensity decreased significantly during the
first five minutes of solvent exposure, and then leveled off (Figure 4.10). The
formation of two Soret Bands, Vi and Vii, makes it difficult, however, to truly assign
the change in absorbance intensities to a removal of chromophores.
The phosphonic acids in the MnP4 would probably have a stronger tendency to
be on the water surface than bound to the porphyrin. Considering the strong tendency
for the Mn-porphyrins to bind water in the axial positions, the chromophore could be
very hydrophilic, promoting their tendency to lay on the water surface. This behavior
was observed in MnP4 films transferred at high MMA. In films transferred at lower
MMA, chromophore aggregation may have kept many phosphonates from binding to
the zirconium network, leaving them available for binding the manganese. Therefore,
the phosphonate ligation was not as reversible when the films were transferred at
I ■ I
After 5 - 20 min
in CHCI3 soxhiet
350 400 450 500 550
Figure 4.10: LB films of MnP4 transferred at A) 15 mN/m and B) 5 mN/m rinsed in
When LB films of MnP4 transferred at 300 A^ molecule"' (0.7 mN m"') were
rinsed only in hot CH3CN, the band at 477 nm (Band Vii) was again observed (Figure
4.1 1). The Soret Band at 464 nm corresponding to phosphonate binding conversely
disappeared. These results suggest that hot solvents were able to eliminate the
phosphonic acid binding leading to the stable five-coordinate MnTPP(Cl) structure,
and that the appearance of this structure is not a result of chromophore aggregation.
1 1 ■ 1
1 , -T--
before rinsina /
\ 5minCH3CN '
fUTj f 1
A/ ' » / '
350 400 450 500
4.1 1 : MnP4 transferred by LB at 0.7 mN m"' and rinsed in CH3CN: A) transferred
from a 0.5 mg mL"' solution.
22.214.171.124. Transfer of MnP4 from a chloride ion-containing subphase . In order
to avoid intramolecular phosphonic acid-manganese binding upon LB transfer,
chloride ions were incorporated into the subphase at a 0.1 M concentration. The
porphyrin was spread and compressed to 4 mN m"' for transfer onto a zirconated
ODPA template. In the spectrum shown in Figure 4.12, Soret Bands were observed at
461 nm and 476 nm. Clearly, there exist domains of five-coordinate MnTPP(Cl) and
six-coordinate MnTPP(Cl)(PA) structures.
350 400 450 500 550
Figure 4.12: MnP4 transferred from 0.1 M [CI"] aqueous subphase at 4 mN m''.
4.2.4. Self-Assembled films of MnP4
126.96.36.199. Self-assembly from pure solvent . MnP4 films self-assembled from
EtOH/HjO (9/1 mixture) or CH2CI2 showed a Soret Band at lower energies than
observed in MnP4 LB films. The X^^^ was now at ca. 463 - 464 nm, which
corresponds to the peak in MnP4 solutions and in LB films after rinsing off
physisorbed and aggregated chromophores. Therefore, this peak has been attributed to
phosphonic acid binding to non-aggregated metallo-porphyrins. When self-assembled
films of MnP4 were rinsed in hot CHCI3, the red Soret Band associated with the
MnTPP(Cl) increased significantly, and the peak associated with the six-coordinate
MnTPP(Cl)(PA) decreased, again proving that hot solvents can remove the
phosphonic acid ligands leaving the chloride ligand intact. :
However, this five-coordinate structure was not rigid. When the films were
left to structurally relax overnight, the peak at 477 nm decreased in intensity and the
blue shoulder became more intense. This reversible behavior indicates that the ligand
environment, which is very sensitive to solvent and heat, is flexible (Figure 4.13).
425 450 475 500 525
• - Wavelength (nm)
Figure 4.13: MnP4 self-assembled from EtOH/H^O and rinsed in CHCI3. The legend
indicates the spectra after rinsing, after being left overnight and the rinsed again over a
three day period.
When these films were rinsed in CH3CN, behavior similar to CHCI3 rinsing
was observed. The band at 477 nm, again, increased and the band at 464 nm
decreased in intensity with time in the hot solvent. Again, if the film was left
overnight, the band intensities reversed (Figure 4.14). Hot CH3CN, therefore, also
eliminated the phosphonic acid ligand and caused the formation of the five-coordinate
MnTPP(Cl). When the film relaxed, the phosphonic acids had a tendency to bind
again to the central metal forming the six-coordinate MnTPP(Cl)(PA) structure.
350 400 450 500 550 600
Figure 4.14: SA MnP4 films with rinsing in hot CHjCN.
All of the described Band Vii intensity increases were observed after rinsing in
hot solvents. If, as was expected, the axial environment of the porphyrin was
associated with the mobility of these anions in solubilized films, the effect should be
even more pronounced while these films were in solution. To study this, MnP4 films
were self-assembled onto substrates that were cut to fit inside a cuvette filled with hot
solvent, and the UV-vis was taken immediately after the solvent was heated. The
results were, as predicted, exaggerations of what was observed when the film was
removed from the solvent. Figure 4.15 shows that after 5 min in hot solution, the ratio
VA^I was ca. 1 .0, and dropped drastically up to 60 min in hot solvent as Band VI
increased in intensity and area. But, immediately after the film was removed from the
solvent, Band VI dropped leaving the VA^I ratio, again, greater than 1 .0.
Figure 4.15: UV-vis response of a SA MnP4 film during rinsing with hot CH3CN.
Polarized UV-vis results were obtained from the self-assembled films of the
MnP4. The chromophore tilt angle was 90° at all self-assembly times. The self-
assembly of the MnP4 was facilitated by the strong binding between the zirconated
phosphonate template and the phosphonic acids on the MnP4 molecules, and the
position of the phosphonic acids on the periphery of the chromophore lends it to lie
flat in the films.
When the MnP4 was self-assembled onto a zirconated ODPA template, and
these films were then rinsed in hot CHCI3 or CH3CN, the Soret Band conversion from
..^ . .
460 nm to 477 nm was consistently observed. However, when these films were rinsed
in hot EtOH, an anomaly occurred. Instead of the increase in the band at 477 nm, as
was so commonly observed in CHCI3 and CH3CN, now a peak to the high-energy side
of the original band at 454 nm grew in intensity. This peak aligns with the band seen
for the MnP4 and MnPO in EtOH solutions, indicating that in the films, domains of the
chromophores bind EtOH and therefore experience mobility in the axial positions
1 ' 1 '
15min '"s ,' '
' / \
' / ' '
' / '
' ' /
' 1 ' 1 ■
1 I -
Figure 4.16: UV-vis of MnP4 self-assembled films before and after rinsing in hot
188.8.131.52. MnP4 Self-assembly from chloride solutions. To avoid phosphonic
acid ligating the porphyrin central metal and to promote the phosphonic acid binding
to the zirconium network, the MnP4 was self-assembled out of a 1.4 x 10"^ M solution
of porphyrin that was 0.1 M in ^BuNH3^ CI". Following a successfiil self-assembly,
the resulting film will appear completely hydrophobic. However, after the SA fi-om a
chloride containing solution, the film did not appear totally hydrophobic indicating a
potentially incomplete self-assembly. However, the ]V[nP4 was clearly present in the
UV-vis. After rinsing in hot CHCI3, the Soret Band at 475 nm was present with a
distinct peak at 460 nm. When these films were studied after a night in the desiccator,
the Soret showed little change in shape. This behavior is consistent with a preference
for the five-coordinate MnTPP(Cl) after rinsing (Figure 4. 1 7). Interestingly, after SA
from a chloride containing solution, no peak was observed at 418 nm representing the
absense of free-base porphyrin. The excess chloride presumably competes with the
ligation of phosphonic acids and prevents their irreversible binding, which
consequently prevents the demetallation of the MnTPP.
■ 1 ' 1
1 ' 1
1 * -t . ■-''
-• -' *'
1 ,\ After rinsing
.1' ', . hot CH^CI^
/ \ / '
i/^^ % / ;'
350 400 450 500 550
Figure 4.17: MnP4 self-assembled from a 0.1 M chloride solution.
As an additional confirmation of the successful tethering of MnP4 to the
zirconated ODPA template, XPS was performed. A clear peak was observed for F,s,
Ni^and Mn2p3 electrons. The Zrj^j and Zrj^j and P2p3 were also observed. From the
XPS it is apparent that the MnP4 has been incorporated into the film and that the fihn
is stable to UHV. ■.;•'."!
Binding Energy (eV)
Figure 4.18: XPS of MnP4 SA film. The insert is an enlarged view of the same
spectrum between 200 and 80 eV.
MnP4 molecules can be successfully incorporated into ultrathin films using both
the LB and SA techniques. From the UV-vis perspective, these films appear very
similar. In each film, there is a characteristic Soret Band. Typically, this band occurs
at 460 - 464 nm, which has been attributed to formation of a six-coordinate
MnTPP(Cl)(PA) structure. Also, a band at 376 nm was assigned to Band VI, and
Band Va at 418 nm was identified as corresponding to the presence of free-base
porphyrins in the films. Further, rinsing in a solvent that can eliminate the
intramolecular binding of phosphonic acid causes the Soret Band to shift to ca. 477 rmi
representing the five-coordinate MnTPP(Cl). This shift is most pronounced in
solutions in which chloride ions have been added indicating that excess chloride
prevents the binding of the phosphonic acid.
The band representing phosphonic acid binding demonstrated some reversibility.
After films of MnP4 were rinsed in CHCI3 or CH3CN, the structure was that of
MnTPP(Cl). When these same films were left to structurally relax, the UV-vis
showed the reappearance of the band representing the MnTPP(Cl)(PA), accompanied
by a decrease in the intensity of the MnTPP(Cl) peak. When the chromophores were
adhered to the zirconated ODPA network by less than four of the phosphonic acid
tethers, the non-bound phosphonic acid groups remained in central metal's vicinity.
Therefore, displacement of the phosphonic acid ligands with hot solvents does not
prevent them from re-ligating as the film conditions change.
The SA MnP4 films have Soret Band absorbance intensities consistently between
•'■ 0.015 to 0.02 absorbance units. This absorbance intensity corresponds to the LB films
transferred at MMA just before the plateau region. When the porphyrin surface
coverage is incomplete, as in the LB films transferred below 5 mN m'' and in the films
self-assembled for very short times, the absorbance intensity was consistently around
or below 0.01 absorbance units. Therefore, depending on the deposition conditions,
the amount of chromophore incorporated into the films is reasonably consistent.
Consistency in the chromophore loading is helpful in employing these films in
Unfortunately, though the Soret Band at 477 nm is usually associated with
chloride binding, Soret Bands around 460 nm can be associated with a number of axial
ligands and chromophore interactions making difficult absolute characterization of the
porphyrin's ligand and aggregation environment. However, ultimately, the ligand
filling the axial position on the chromophore is less important than its lability. If the
metal center of the porphyrin is available for oxidation, the catalyst will be active.
Also, as will be discussed in Chapter 5, if the imidazole can displace the axially bound
ligand, it can positively influence the catalysis. ■ *
INCORPORATION OF AN IMIDAZOLE LIGAND INTO MANGANESE
PORPHYRIN CONTAINING ZIRCONIUM PHOSPHONATE THIN FILMS
As in Chapter 4, the chromophore catalyst studied was a tetraphenyl porphyrin
para-substituted with four octadecylphosphonic acid groups (MnP4), which could
tether the porphyrin directly to the zirconium surface. For comparison, the model
porphyrin MnPO with no phosphonic acid chains was also examined. The heterocyclic
ligand used for these experiments was an alkylphosphonic acid imidazole (ImODPA),
which could also be easily attached to the zirconium surface. From the propensity of
the imidazole to protonate in the presence of HBr during the ImODPA synthesis, the
imidazole unit was the bromide salt upon film preparation (Figure 5.1). Deprotonation
of the imidazole was attempted in order to facilitate its binding to the metallo-
porphyrin. To study the porphyrin's UV-vis sensitivity to an imidazole ligand in
solution, an imidazole with no alkyl chains (ImH) was also used for solution studies.
The anticipated structures of the mixed MnP4/ImODPA and MnPO/ImODPA
films are shown in simplified form in Figure 5.2. The chromophore is at the exterior
of the film and available to catalyze the reaction of interest. The bulky chromophore
leaves vacant sites available for the ImODPA, which is tethered to the zirconium-
phosphonate network under and around the chromophore (Figure 5.2). Multiple
ImODPA are anticipated to be located below the chromophore plane; however, only
one has the opportunity to bind at a time to the metallo-porphyrin core.
A 0(CH2)i8PO(OH)2 (R)
F^ A. /F
Figure 5.1: Structures of A) MnP4, B) MnPO, C) ImODPA and D) ImH.
Figure 5.2: Schematic of MnP4 and ImODPA films.
A variety of methods were employed in order to accommodate the porphyrin's
large MMA and encourage its binding to the imidazole in the films. These formation
procedures, which were motivated by a number of intentions, will be described in this
chapter. First, the Mn-porphyrin/imidazole binding is in an equilibrium state. In order
to encourage the imidazole binding, or at least its availability for binding, the
imidazole had to be present in excess relative to the porphyrin. To insure excess
imidazole, this layer was often prepared first by either the SA or LB method. The
hydrophilicity of both the phosphonic acid and the imidazole chain terminating groups
of the ImODPA made this molecule impossible to transfer as pure LB films.
However, the ImODPA could be mixed with a good film-forming molecule such as
ODPA or HDPA (hexadecylphosphonic acid), and these LB films were transferred
The film preparation methods fall into two general categories. First, the two
components, ImODPA and MnP4, were assembled by either LB or SA in two separate
steps. Examples include mixed ImODPA/HDPA LB films onto which MnP4 was self-
assembled, ImODPA SA films followed by MnP4 SA, and MnP4 LB or SA films with
ImODPA SA in the second step. The mechanism of the second SA step is less
straightforward than the first. The phosphonic acid tethers on the second amphiphile
must either find vacant or defect sites in the zirconium network, or actually replace
already existing Zr-phosphonate bonds. Because this step is not a true self-assembly,
it will also be referred to as the "substitution" step. The second category of film
preparations involves the MnP4 and ImODPA being assembled in one step, either as a
mixed SA or LB transfer. All of the described methods were successful in
incorporating both components to some degree. The most facile involved SA of
ImODPA followed by substitution of MnP4.
Following the formation of the ImODPA film, MnP4 could be substituted from
CHCI3, EtOH, or EtOH/water. Immediately after substitution, some MnP4s were
physisorbed to the surface and were easily removed by rinsing in a hot solvent. The
chromophores that remained contained at least one phosphonic acid tether bound to
metal phosphonate base. Film stability was followed by UV-vis, and after the first 5
min of exposure to hot solvents, no more significant chromophore loss was observed.
One goal of this project was to understand the propensity of the imidazole
ligand to bind to the porphyrin and to determine certain characterization signatures to
confirm that this binding had taken place. That the imidazole/porphyrin binding be
persistent in the films is less important than that the imidazole is present and available
5.2 Solution Studies
Ligand effects as well as chromophore orientation and aggregation are
primarily reflected in the shape or shift of the Soret Band or Band V. Therefore, the
behavior of this band was carefully monitored. In addition, the ratio of Bands V to VI
could be observed resulting from a change in the ligand environment. "^ Observed
changes in the Soret Band in a non-coordinating solvent solution of a potential ligand
are partially due to polarizability of the ligand. High polarizability of the axial ligand
will lead to less negative charge induced on the porphyrin through the metal, and the
Soret will shift to lower energies. • ^^
As mentioned in Chapter 4, the MnP4 spectrum demonstrates a strong
tendency to intramolecularly bind to the phosphonic acid head-groups on the alkyl
chain substituents. Because the imidazole binding is crucial in the catalysis studies, it
was important to consider the ability of imidazole to replace phosphonic acid ligands
in these systems. According to Arasasingham, the displacement of a water or hydroxy
ion by imidazole is favored,"'* however, the ability of imidazole to replace
phosphonate ligands had no literature precedence.
5.2.1 MnPQ and MnP4 with ImH ' "* ^ '' '^ - ' -^' '
To determine the electronic effects of binding the imidazole in the absence of
potential phosphonate ligands, an imidazole containing no long phosphonic acid chain
(ImH) was added to a solution of MnPO, where the concentration of the MnTPP was
held constant and the ratio of ImH:MnTPP was increased. With MnPO, as an excess
of the imidazole was added, an obvious blue shift of about 20 nm to ca. 458 nm was
observedinthe A,n,ax(Figure 5.3A). ■ \ . • • ,'-\
-1 ■ r
Figure 5.3: Solvent response of A) MnPO and B) MnP4 to ImH.
Upon addition of ImH to MnP4 solutions in CH2CI2, the original red shoulder
on the Soret Band disappeared and was followed by a blue shift of the A-^ax to 458 nm
(Figure 5.3B). This peak was in reasonable alignment with the peak observed in the
MnPO solutions under the same conditions. Imidazole ligands are not reported to
encourage aggregation effects; therefore, these results again imply that the blue shift
represents imidazole binding. This blue shift corresponds to a similar shift reported by
Bruice et. al in their solution studies of a Mn-porphyrin with an increasing
concentration of an imidazole ligand. ' "
At the high concentrations of ImH relative to porphyrin, the imidazole filled
either one or both of the axial positions on the metallo-porphyrin. The imidazole,
therefore, successfully displaced the phosphonic acid. Also, in the solutions
containing excess ImH, Band Va, which was assigned to fi"ee-base porphyrin, is
reduced. With the competition between imidazole and phosphonic acid for the axial
position, the phosphonic acid is obviously less likely to demetallate the Mn-porphyrin.
At lower imidazole concentrations, the blue shift was less obvious. However, the
effective concentration of phosphonic acids on the MnP4 in the vicinity of the metal
was very high, so the solution concentration of ImH had to be large to compensate.
5.2.2. MnP4 and MnPO with ImODPA
When ImODPA was added to MnPO in CHCI3, the blue shift indicating
imidazole or phosphonic acid binding was not observed. Instead, the Soret Band
shifted from 477 nm to 483 nm. This red shift in the Soret Band is due to bromide
displacing chloride as the axial ligand. Bromide originates from the ImODPA, which
is originally a protonated amine-bromide salt. Mn-porphyrins with bromide ligands
absorb at higher wavelengths than with chloride ligands because bromide is less
electronegative and more polarizable than chloride shifting the Soret Band to lower
energies.' 15 a close look at the region ca. 41 8 nm shows that Band Va is also
appearing. This band has been attributed to the presence of free-base porphyrins.
which again, exist due to an irreversible binding of the phosphonic acid to the
manganese central metal under certain conditions causing the demetallation of the Mn-
400 450 500
Figure 5.4: Solvent response of A) MnPO and B) MnP4 to ImODPA. Legends
indicate the molar ratio of MnP to ImODPA.
ImODPA was also added to a CHCI3 solution of MnP4. The spectral behavior
of this solution was more unusual. With 1 eq. of ImODPA, the red shoulder appeared
at 483 nm and the blue shoulder at 463 run was reduced in absorbance intensity. This
spectral behavior corresponds to some of the phosphonic acid ligands being displaced
by bromide. However, the bromide ligation did not appear to be quantitative, as was
the case with the MnPO. After the addition of 10 eq. of the ImODPA, the intensity of
the peak at 483 nm leveled off, and the 463 nm peak was at a minimum. With 20 or
more equivalents of ImODPA, the blue Soret peak, or Band Vi, shifted to 460 nm and
increased in intensity (Figure 5.4). This blue shift may indicate imidazole binding.
Since the ImODPA amphiphile is terminated by a phosphonic acid, it would
seem, again, that addition of this amphiphile would only serve to increase phosphonic
acid binding. The competing ligands, however, make definitive identification of the
peak at 460 nm difficult. Above a certain ImODPA concentration, it appeared that the
imidazoles were displacing the remaining phosphonic acids and possibly some
bromides and shifting Band Vi to higher energies. Therefore, a MnP(Br)(Im) species
was probably present in solution at high ImODPA concentrations. In the films of the
Mn-porphyrin, this competition may be avoided if the phosphonic acids are bound to
the zirconated ODPA template.
5.3 Film Studies
5.3.1 . Langmuir-Blodgett Films containing substituted MnP4
184.108.40.206. MnP4 substituted into an HDP A LB film . To understand the ability of
the porphyrin to bind to the zirconium network in a preformed bilayer, a film was
prepared by substituting a MnP4 layer into a LB film of hexadecylphosphonic acid
(HDP A). By substituting the porphyrin film into an aliphatic capping layer, the hope
was that the phosphonic acids could secure the porphyrin to the surface while the
HDPA would be able to prevent porphyrin aggregation. The HDPA film formed in
this manner was well-organized and represented the most compact, and therefore most
challenging, bilayer for MnP4 substitution.
With the potential phosphonic acid binding sites in the Zr-network occupied
with HDP A, the phosphonic acid tethers on the porphyrin were available for
intramolecular ligation. From solution results, however, chloride or bromide could
displace these phosphonic acid axial ligands (Figure 5.4). When the substituted film
was studied by UV-vis immediately after the MnP4 SA process was complete, the A,,,,^
appeared at 454 nm. At such a high energy, the Soret Band implied that the
chromophores were aggregating. Two lower energy shoulders could also be
identified in the broadened Soret Band. The first appeared at ca. 463 nm, which may
represent non-aggregated porphyrin domains, and the second appeared at ca. 478 nm at
lower intensity indicating the presence of MnTPP(Cl) moieties.
The film stability was examined by exposure to hot CHCI3 and a loss in
absorbance intensity and a significant peak broadening was observed. After the first 5
min in hot CHCI3, no ftirther decrease in the overall absorbance intensity of the film
was perceived, and this spectrum represented only the chemisorbed porphyrins. With
the physisorbed chromophores removed, the original red shoulder at 478 nm, which
was also observed previously in the solution and films of pure MnP4, was more
distinct, and there appeared to be two peaks. Bands Vi and Vii. From the two apparent
Soret Bands, it seems that the remaining porphyrins were either MnTPP(Cl)(PA) (peak
at 463 nm) or MnTPP(Cl) (peak at 478 nm) (Figure 5.5).
Due to the splitting and broadening of the Soret Band, it is impossible to
attribute a change in absorbance intensity just to removal of physisorbed
chromophores. Some physisorbed chromophores were indeed removed, which was
found by calculating the differences in peak areas before and after rinsing. Figure 5.5
also reveals a decrease in the absorbance intensity ratio of Band V to Band VI. A
change in the ratio of Band V to Band VI is often reportedly associated with a change
in the ligand environment^^-^i Unfortunately, many possible ligand changes are
present in these systems. Most likely, as the film was rinsed, the chloride ligands
could have displaced the phosphonic acid ligands. However, from the solvent studies
of the pure MnPO with ethylphosphonic acid described in Chapter 4, a decrease in
VA^I was associated with increased phosphonic acid binding. Therefore, trends in
these ratios were difficult to follow because so many scenarios were possible.
350 400 450 500 550
Figure 5.5: UV-vis of ODPA/Zr/HDP A, SA MnP4 film rinsed in hot CHCl,
220.127.116.11. MnP4 substituted into an ImODPA/HDPA LB film. Though HDPA
was easily transferred onto the Zr-ODPA template using the LB method, imidazole
was much more difficult to deposit in this manner. The hydrophilic nature of both the
phosphonic acid and the imidazole made this molecule unsuitable for Langmuir
monolayer formation. Therefore, the imidazole was diluted with HDPA or ODPA to
avoid this problem. After the LB transfer of the ImODPA:HDPA mixtures, the MnP4
was substituted into this layer. When films containing solutions ranging from 10% to
75% ImODPA in HDPA were prepared and the MnP4 was substituted into these films,
a pattern was observed (Figure 5.6). In the films of 10% ImODPA, SA MnP4, the
ratio of VA^a was clearly less than 1 .0. This ratio increased with an increased
concentration of ImODPA. Band Va, associated with the free-base porphyrin, drops in
intensity with the addition of an imidazole ligand to compete with the phosphonic
acid, as was seen in solution studies with MnP4 and ImH (Figure 5.3B). The
presence of free-base porphyrin is possibly due to the irreversible binding of
phosphonic acid to manganese that then pulls the manganese out of the center of the
chromophore. Therefore, if the imidazole prevents the phosphonic acid from binding,
it may preserve the integrity of the metallo-porphyrin.
Likewise, though less dramatic, the ratio of VA'^I also appeared to increase
with added ImODPA. Unfortunately, the ratio of VA^I in these films was not always
reproducible or controllable, and this ratio fluctuation was observed in films when
imidazole was absent. These inconsistencies could be due to the inability to
completely direct the axial binding of the metallo-porphyrins.
The Soret Band in Figure 5.6 demonstrates splitting after rinsing in hot CHCI3.
The blue side of the peak occurs at 462 nm, which is slightly red shifted from the peak
previously attributed to imidazole binding. However, as the percentage of imidazole
increases, Band Vi appears to shift slightly to the blue. Also, the occurrence of the
second, red band at 477 nm indicates the presence of halide axial ligands assigned to
MnTPP(Cl). Due to the steric constraints from the film environment, it is unlikely that
these porphyrins would form bis-imidazole complexes [MnTPP(Im)2]. Therefore, this
peak splitting has been assigned to the formation of asymmetric axially bound
porphyrins coexisting with domains of MnTPP(Cl).
- - 25% imid
. . . in% imirl
* • • lU /o UMIU
• v '
. V" ^
■ * *^JZJ" *** ^ -
350 400 450 500 550
Figure 5.6: MnP4 substituted onto ImODPA:HDPA LB films after CHCI3 rinsing.
Rinsing in hot CHCI3 successfiilly eliminated any physisorbed chromophores.
However, hot CHCI3 had a tendency to also cause a change in the ligand environment.
Therefore, room temperature CHCI3 was examined in order to eliminate the
physisorbed chromophores. After rinsing a 25% ImODPA/HDPA LB film, MnP4 SA
film in room temperature CHCI3, the UV-vis showed behavior different compared to
hot CHCI3. The absorbance intensity of the Soret Band decreased along with a shift
from 456 rmi to 458 nm (Figure 5.7).
1 hr in RT CHCI,
350 400 450 500 550
Figure 5.7: MnP4 substituted onto a 25% ImODPA/HDPA film, rinsed in room
temperatvire and hot CHCI3.
When the film was then exposed to hot CHCI3, the absorbance intensity
decreased further and the red component of the Soret Band appeared at 477 nm.
Therefore, physisorbed chromophores can be eliminated from the surface by using
room temperature or hot solvents, but a change in the axial environment was only
associated with rinsing in hot solvents. The band representing halide binding at ca.
477 nm was not permanent and relaxed back to the original peak at ca. 460 nm after 24
hr (Figure 5.8).
Figure 5.8: UV-vis of an ImODPA/ MnP4 film after drying.
18.104.22.168. MnPO linked to ImODPA containing LB films. MnPO, a porphyrin
with no alkylphosphonic acid chains, was assembled into ImODPA SA films to study
the imidazole binding in a simplified system. The only available methods of
absorbing MnPO to these films were through the imidazole or through aggregation of
chromophores, i.e. physisorption, to already chemisorbed porphyrins. Upon rinsing,
any physisorbed MnPO's should be washed away. Figure 5.9 shows the UV-vis of a
layer of MnPO attached to a 25% ImODPA/HDPA LB film. The absorbance intensity
of the MnPO film was much less than films containing the MnP4, but the porphyrin
was clearly present indicating the imidazole binding was active. After 60 min in a hot
CHCI3 solution, the absorbance intensity of the film did decrease indicating
chromophore loss. However, the imidazole binding was somewhat stable to hot
CHCIj^ as the chromophores were not completely removed after 60 min in the solvent.
The ratio of ImODPA to HDPA has been varied, and MnPO films were still
successfully formed. Further, using an ImODPA/ODPA mixture instead of
ImODPA/HDPA also worked to prepare the MnPO films. The 25% ImODPA films
were studied primarily because of the balance of high imidazole loading and stable
film behavior. HDPA was the primary diluent used because its chain length was
appropriate to form a film with the imidazole group fully exposed and available for
550 400 450 500 550
Figure 5.9: MnPO attached to a 25% ImODPA/HDPA LB film and rinsed in hot
5.3.2. Mn-porphyrins substituted into self-assembled films of ImODPA
22.214.171.124. UV-vis response of MnP4 film to /-butyl amine and CHCU rinsing.
MnP4 was successfully substituted into a SA layer of ImODPA. After substitution,
the films were rinsed with a solution of ^butyl amine (/-BuNHj) to deprotonate the
imidazole and facilitate its binding to the porphyrin. The base /-BuNHj was chosen
for its bulky nature, which should prevent its binding to the porphyrin. Rinsing the
films in /-BuNHj caused a blue shift in the Soret Band from 461 nm to 458 nm, which
was therefore, assigned to imidazole binding to the central metal. It was unclear if
imidazole binding was encouraged by deprotonation or if the elimination of some
physisorbed and therefore, non-imidazole bound porphyrins made it easier to observe
the imidazole binding. Because of the film structure, the complex is most likely a 6-
coordinate MnTPP(Cl)(Im) system.
After rinsing with hot CHCI3, the Soret absorbance intensity decreased
significantly. Band Vii shifted to 477 nm, which was again associated with the
formation of the five-coordinate MnTPP(Cl). Twenty-four hours after rinsing, the
halide-binding peak at 477 nm was no longer detected. Instead, the band at 465 nm
increases in intensity as the films were left to reorganize overnight as was observed in
the pure MnP4 films. From the energy of the band at 465 nm, which is at lower
energies than the Soret Band observed with imidazole binding, this peak was likely
due to the binding of phosphonic acid ligands and not imidazoles (Figure 5.10). The
Soret Band absorbance in this film was comparable to that of previous examples of
pure MnP4 substituted imidazole films, implying roughly the same number of
chromophores were present.
-•— rinse CHCUhot) .
400 450 500
Figure 5.10: MnP4 substituted onto a pure ImODPA SA film.
126.96.36.199. Reversibility of the chloride binding in MnP4 substituted films. The
same reversibility of the halide binding was observed in the films of MnP4 substituted
into SA ImODPA films. As shown in Figure 5.11, after 24 hr, the peak at 477 nm
reduced in intensity while the peak at 460 nm increased in intensity. The halide
binding could be achieved again by rewetting this film, and the cycle continued for up
to a week. However, with each cycle, the halide binding became slightly more
persistent. The reversibility of the binding, as observed by the shifts in the Soret
Bands, indicated that the molecular behavior in these ultrathin films was not static. In
fact, in thin films, the porphyrins were keenly sensitive to the environment, especially
environments containing potential axial ligands.
■ hot CHCI3
Figure 5.1 1: Reversibility of the chloride/phosphonic acid binding.
-• 188.8.131.52. Rinsing the substituted MnP4 films in chloride ion solutions . Chloride
ions were deliberately added to the system by rinsing the films in a solution of tert-
butylammonium chloride (r-BuNHj^ CI"). These results confirmed that the peak at 477
nm in the films represented the MnTPP(Cl). When the MnP4 substituted film was
rinsed in a room temperature EtOH solution of chloride ions, there was a clear Soret
peak splitting with the red peak occurring at 477 nm indicating that in excess, chloride
can bind at room temperature. When this film was then placed in a room temperature
chloride solution in 9/1 EtOH/H20 (the SA solvent mixture), Band Vii disappeared
and Band Vi intensified (Figure 5.12).
§ 0.004 H
RT CI- (EtOH)
RT CI- (EtOH/HjO)
A, CI-, (EtOH/HjO)
■A— overnight (above)
Figure 5.12: MnP4 substituted film rinsed in chloride and t-butylamine solutions.
To fiirther promote chloride binding, the chloride-containing EtOH/H20
solutions were heated. Heating the solution worked remarkably well at incorporating
chloride binding, and the low energy peak developed significantly. After being left
overnight, the red peak was much more persistent. This result may indicate that in the
presence of a large excess of chloride ions, chloride binding is much less mobile and
more difficult to displace (Figure 5.12).
Rinsing in /-BuNHj in room temperature EtOH caused the complete and
immediate displacement of the chloride ligands. It is known that imidazoles tend to
bind more efficiently to the porphyrins than chlorides, and so it is possible that the
band at 462 nm represents imidazole binding." '
184.108.40.206. Preparing the substituted film from a chloride containing solution . To
avoid phosphonic acid binding upon film formation, a layer of MnP4 was assembled
from a 0.1 M solution of chloride onto an ImODPA layer (Figure 5.13). Initially, the
chloride binding was not obvious, and the Soret was slightly blue shifted relative to
phosphonic acid binding.
-l ■ 1 •-
350 400 450 500
Figure 5.13: MnP4 substituted from a 0.1 M CI" solution onto an ImODPA layer, and
compared to a MnPO solution with ImH binding. , .
- , /• :
^ » ,. 130
After sitting overnight, the red chloride peak again emerged. This behavior is
contradictory to results obtained previously where the chloride peak seemed to
disappear with time. By adding a sufficient amount of chloride to fill the porphyrin
axial positions, the phosphonic acids may be forced to bind to the zirconium network.
The chloride axial binding is in equilibrium while the zirconium-phosphonate binding
is not. Once the phosphonates are unavailable for ligation, the imidazole binding,
which is evidenced by a blue shifted Band Vi, is more likely to be observed. Further
proof of imidazole binding is in Band Va, which was earlier associated with
phosphonic acid binding causing the demetallation of the Mn-porphyrin. This band
was clearly absent in these films.
5.3.3. Self-assembling the MnP4 and ImODPA from a mixed solution
As an alternative method, a self-assembly solution of a 70/30 mixture of the
ImODPA and MnP4 was prepared. The UV-vis of this mixed SA solution showed
Band Vi at 460 and Band Vii at 477 (Figure 5.14). Band Vi in solution may represent
phosphonic acid binding; however, as the film was formed these phosphonic acids
were attracted to the zirconium network leaving the manganese available for binding
the imidazole. The blue shift may also demonstrate some contribution from porphyrin
aggregation. Rinsing with a hot solvent was consistently done to remove any
physisorbed chromophores from these films, though in some cases, as shown in Figure
5.14, there often was very little loss in absorbance intensity overall indicating that
there were few physisorbed chromophores present initially.
after SA A- . -
-.•■■. / ■ V
\ after 15 min
Figure 5.14: ImODPA/MnP4 self-assembled from 70/30 mixture and rinsed in hot
5.3.4. Other methods for preparing ImODPA and MnP4 containing films .
5.3.4. 1. LB deposition of MnP4 followed bv substitution of ImODPA . Films
of MnP4 were transferred from a water subphase at 5, 10, and 15 mN m"' (before,
during and after the plateau region of the isotherm shown in Figure 4.7). ImODPA
was substituted into these films from an EtOH/H20 solution. It was thought that, if
the MnP4 could form a complete monolayer with phosphonic acids bound to the
zirconium network prior to imidazole being present, it may be more likely that there
would be imidazole binding instead of phosphonic acid binding the central metal. The
UV-vis behavior of these films after the substitution of ImODPA and after rinsing
with CH2CI2 at room temperature is shown in Figure 5.15. Unfortunately, there are
many complications with the films prepared in this way. First, the imidazole will most
likely want to bind to the exposed surface of the porphyrin first; however, the
phosphonic acid will be trying to bury into the film to find the zirconium network.
Further, it is very difficult to confirm the presence of the imidazole in these films by
any means. Therefore, this method was not rigorously pursued.
Rinse RT MeCI^
400 450 500 550
Figure 5.15: ImODPA substituted into a MnP4 LB film transferred at 10 mN m"'.
220.127.116.11. LB transfer of mixed MnP4/ImODPA film at high dH. For
comparison, a LB film was prepared from a spreading solution containing MnP4 and
ImODPA in a 1 to 3 ratio. Ideally, a monolayer richer in ImODPA would have been
used, but again, this is a poor amphiphile for LB films, and so this was impossible.
These films were transferred at a variety of pressures and pHs. In a film transferred
before the onset of the isotherm at a pH of 1 1.3 (Figure 5.16), the Soret Band after was
at 459 nm. When the film was transferred at a MMA associated with little
chromophore aggregation, the Soret Band at 459 nm possibly corresponds to
imidazole binding. The basic subphase was intended to deprotonate the imidazole
and encourage its binding to the porphyrin while reducing the porphyrin aggregation,
as seen in the case of the Pd-porphyrins in Chapter 3. Additionally, the basic subphase
would deprotonate the phosphonic acids causing them to be even more hydrophilic -
and more likely to be on the water surface. Unfortunately, it is very difficult to
confirm the presence or binding of the imidazole in these films. XPS scans of the Nj^
region were not helpful in determining if there were two nitrogen environments
present; therefore, these films were not a major focus for the preparation of catalytic
400 450 500 550
Figure 5.16: LB film of MnP4/ImODPA transferred from a 25/75 mixture on an
aqueous subphase, pH 11 .3. a
5 -*•« I
5.3.5. Characterization of films containing MnP4 and ImQDPA bv XPS and ATR-IR
18.104.22.168. XPS results of pure MnP4 and pure ImODPA films . Though the UV-
vis is the most well documented method of characterization for porphyrin films, the
ImODPA had no clear transition in the UV-vis. Therefore, to characterize the
ImODPA self-assembled film prior to the MnP4 substitution, ATR-IR and XPS were
employed. To study the formation of the ImODPA films by XPS, a layer of ImODPA
was self-assembled onto a zirconated ODPA template on a silicon wafer.
405 400 395
Binding energy (eV)
Figure 5.17: XPS multiplex scan over the N,, region of A) ImODPA, and B) MnP4
self-assembled films. The dashed line represents the Gaussian peak tit.
The layer of pure imidazole was scanned by the multiplexing technique over the
nitrogen peak (N,J, between 410 and 390 eV. The nitrogen peak, which could only be
due to the presence of imidazole, was clearly a single Gaussian peak centered at 401.0
eV with a FWHM of 2 eV (Figure 5.1 7 A).
When a capping layer of pure MnP4 was self-assembled onto the zirconated
ODPA template, a nitrogen peak was also seen (Figure 5.17B). However, in these
films, the nitrogen peak was broadened to 3 eV and shifted to 399.0 eV. The nitrogen
atoms of the imidazole and the porphyrin are in two different envirormients, and a
small change in their binding energies would be expected. Therefore, in the mixed
film, two peaks should be present in the N,s region, one representing the imidazole and
one representing the porphyrin.
22.214.171.124. XPS results from mixed self-assembled films . A film SA from a
70/30 mixture of ImODPA and MnP4 in CH2CI2 was studied by XPS to determine if
two nitrogen environments could be detected. There was a strong peak at about 402.0
eV and a distinct shoulder or second peak of weaker intensity at 399.0 eV indicating
the presence of both nitrogen environments, and hence of both the imidazole and
porphyrin within the films (Figure 5.18). Alternatively, a mixture of the porphyrin and
imidazole in a similar ratio was self-assembled from 9/1 EtOH/HjO. The nitrogen
region of these self-assembled films showed two peaks, the first centered at 402.0 eV
and the second, a broader peak of approximately equivalent intensity centered at ca.
400.0 eV (Figure 5.19). These peaks align reasonably well with those of the XPS
studies of the pure imidazole (40 1 .0 eV) and porphyrin (399.0 eV).
Again, these results indicate the presence of both the imidazole and the
porphyrin in these self-assembled films. Similar results were observed by Offord et al.
in films of ruthenium or osmium metallo-porphyrins that were adhered to a SA thiol-
on-gold surface.30 XPS, therefore, clearly demonstrated the presence of both the
porphyrin and the imidazole in the films.
405 400 395
Binding Energy (eV)
■■■. .•■ *i
Figure 5.18: XPS multiplex scan of Nls region of ImODPA/MnP4 film self-
assembled out of 70/30 CH2CI2 solution. The dashed lines represent the Gaussian
410 405 400 395
Binding Energy (eV)
Figure 5.19: XPS multiplex scan of ImODPA/MnP4 film self-assembled from a 70/30
mixture in EtOH/HjO. The dashed and dotted lines represent gaussian peak fits.
126.96.36.199. ATR-IR characterization of the films containing substituted MnP4 on
a self-assembled ImODPA layer. An ODPA monolayer transferred by the LB
technique onto a Zr-ODPA template leads to alkyl peaks at 2918 cm-i and 2850 cm-i
corresponding to the Va(CH2) and Vs(CH2) stretches, respectively. For alkyl chains in
a majority /ra«5-configuration with crystalline order, the Va(CH2) peak usually falls
between 2918 cm"' and 2920 cm-'. A shift to higher energies indicates the
introduction of disorganization within the hydrophobic region of the film. In addifion,
the full width at half maximum (FWHM) of the Va(CH2) peak can be a measure of the
packing and conformational order within the alkyl region. An organized, close-packed
film has a FWHM of approximately 17 cm"', which can stretch to 35 cm*' upon
disorganization of the film.
In films formed by both the LB and SA of ODPA fi-om a 9/1 EtOH/HjO
solution onto a zirconated ODPA template, the Va(CH2) stretch comes at 2918 cm-'
and the Vs(CH2) is at 2850 cm-'. Therefore, these films are mostly trans and close-
packed. In addition, the absorbance intensity of the Va(CH2) peak in the capping layer
is between 0.012 and 0.015 au. An alkyl absorbance intensity of this magnitude is
therefore associated with the formation of a complete, organized monolayer of an
In order to further confirm the presence of imidazole in the films formed by the
SA of ImODPA onto a zirconated ODPA template, the Va(CH2) and Vs(CH2) peaks
were monitored and compared to the ATR-IR results obtained from the ODPA
monolayer. The ATR-IR spectra of the ImODPA films were referenced to the
zirconated ODPA template; therefore, what was plotted were the results from just the
ImODPA capping layer. When the Zr-ODPA template had soaked in a 9/1 EtOH/H.O
\- \: ■ :■ ' -i^W-^
solution of ImODPA for five minutes, the absorbance intensity of the Va(CH2) peak
was 0.004 au, and the Va(CH2) and Vs(CH2) peaks appeared at 2926 cm-' and
2854 cm"i, respectively. After 20 min, the absorbance increased to 0.0065 au and the
peaks shifted to 2920 cm"' and 2850 cm"'. There was no further shift observed in
these peaks; however, the absorbance slowly increased over 2 hr, at which time the
absorbance intensity remained constant at 0.012 au. These results proved the presence
of a monolayer of ImODPA (Figure 5.20 and 5.21). However, the FWHM of 28 cm''
was significantly greater than those seen in ODPA monolayers, indicating a level of
disorganization in these self-assembled films. A peak was also observed at 1307 cm'',
which corresponds to the presence of C=N stretches from the imidazole. Hence, ATR-
IR studies confirm the presence of a monolayer of ImODPA formed from the SA of
this amphiphile fi"om EtOH/HjO.
v(CH,) = 2918cm'
Figure 5.20: ATR-IR of ImODPA SA film.
■Q 0.006 H
100 150 200
Figure 5.21: Increase in absorbance intensity of 2918 cm"' peak in ImODPA with SA
After a layer of pure ImODPA was formed, a layer of MnP4 was self-
assembled to monitor the incorporation of the porphyrin into these films. After 1 hr,
, the absorbance intensity of the Va(CH2) peak leveled off at 0.016 au (as referenced to
the ImODPA monolayer), but the peak consistently appeared at 2924 cm-'. With such
a large chromophore on the exterior of the hydrophobic region, the alkyl groups would
be expected to be more dispersed and hence, show significantly lower absorbance
intensity. Also, the addition of these bulky amphiphiles would be expected to increase
the disorganization of the monolayer and shift the alkyl peaks to higher energies.
However, the absorbance intensity implies a relatively high density of alkyl chains
fi-om the MnP4 units with only slightly more disorganization than seen in SA ODPA
films. The physisorption of the MnP4s could result in the alkyl chains burying into the
hydrophobic region of the chemisorbed film, leading to a high density of alkyl chains
along with some semblance of organization within this region. The possible
physisorption of a number of chromophores is expected due to the propensity of the
porphyrins to aggregate. However, this impHes a significant reorganization of the
underlying imidazole monolayer in order to have room to incorporate these alkyl
After 30 min in hot CHCI3, the absorbance intensity leveled off at 0.01 1 au and
Va(CH2) peak occurred at 2928 cm-', as referenced to the imidazole monolayer (Figure
5.22A). This result indicates that the true porphyrin containing monolayer contains
alkyl chains that are disorganized and include a number of gauche interactions. After
removing the physisorbed chromophores, the true MnP4 monolayer characteristics
were better defined.
Also, the SA of this film for only 5 min, leading to an absorption intensity of
0.004 au, formed a SA film containing approximately 30% of a complete imidazole
monolayer. This incomplete film was then exposed to a MnP4 solution. After 30 min,
the absorbance intensity leveled off at 0.018 au and the Va(CH2) was at 2926 cm-'.
After only 10 min in hot CHCI3, the absorbance dropped drastically to 0.005 au and
Va(CH2) shifted to 2937 cm"' (as referenced to the imidazole monolayer). However,
the IR shows that the SA of a 30% ImODPA layer resulted in a relatively poor
monolayer, which left many defect sites. Originally, it was thought that these defect
sites would allow for the more straightforward inclusion of the porphyrin.
Unfortunately, the imidazole layer in such a state may be easily removed fi-om the
zirconated ODPA template, and the final film may include less imidazole and
porphyrin (Figure 5.16B), and therefore, have a lower overall alkyl absorbance in the
3100 30'00 2900 2800
3000 2900 2800 2700
Figure 5.22: ATR-IR of alkyl region of: A) MnP4 substituted on a 100% ImODPA
base capping layer, B) MnP4 substituted on a 25% ImODPA base capping layer.
Thin films containing both the MnP4 and the imidazole on the surface of a
zirconium phosphonate film have been successfully prepared by a variety of methods.
The imidazole is present in the films for the purpose of acting as a heterocyclic ligand
to promote the catalytic behavior of the MnP4 chromophore. Because of the
flexibility of assembling the zirconium phosphonate thin films, the procedure for
preparing these films has been varied to optimize the incorporation of ImODPA and
MnP4. Though aggregation is difficuk to identify due to the strong optical influences
of the different axial ligands, the evidence indicates that the aggregation is probably
not significant in these films.
Originally the spectral changes observed with rinsing were considered due to
some chromophore reorganization or possible solvent effects. However, the peak at
477 - 478 rmi aligns with the porphyrins' solvent UV-vis spectra only in the presence
of added chloride or bromide; therefore, halide binding has been concluded to cause
this red peak in the films as well. As mentioned, the chloride or bromide ligand
causing the Soret Band change is probably associated with the MnP4 as the original
axial ligand or bound to the imidazole through the HBr sah. When solubilized in hot
solvents, the halides then have the opportunity and the preference for binding at the
porphyrin metal center. The halide binding can be seen after rinsing in hot CHCI3 or
hot CH3CN, as shown in Figures 4.1 1 and 4.14.
From ATR-IR, XPS, and UV-vis, the existence of ImODPA and MnP4 within
the films is confirmed. From the ATR-IR, the formation of the imidazole-containing
capping layer was determined. In addition, UV-vis and XPS confirms the presence
and behavior of the pure MnP4 capping layer. XPS results show a nitrogen
environment within the porphyrin, which differed in binding energy fi-om the nitrogen
peak for a pure imidazole film. In the mixed films, the ATR-IR demonstrated the
addition of porphyrin alkyl and aryl environments upon substitution into the
The ability of the MnP4 films to reversibly bind different axial ligands is
inferred from the activity in the Soret region of the UV-vis spectra upon heat and
solvation. It appears that the Soret Band splits into two peaks, which typically
represent the binding of a chloride ligand and the binding of either a phosphonic acid
or an imidazole ligand in the fifth and sixth coordination sites. Further, it is possible
that porphyrin domains form with slightly different ligand environments.
From solvent studies, it is clear that the imidazole can displace either
phosphonic acid or chloride ligands if present in a high enough concentration. This
binding is in equilibrium, making this again, difficult to consistently observe in the
UV-vis. However, it is believed that in the case of the catalysis reaction, the binding
will be favored due to the excess of imidazole present and that this binding will cause
the activation of the porphyrin for catalysis using peroxide oxidants even if it is not
consistently observed by UV-vis.
MANGANESE PORPHYRIN AND IMIDAZOLE CONTAINING
ZIRCONIUM PHOSPHONATE THIN FILMS AS CATALYSTS
Reactions using immobilized catalysts have recently gained much interest. First, the
immobilized porphyrins are somewhat protected from destructive oxidation because they
are ideally isolated from one another on the surface. Elimination of destructive oxidation
can improve the catalyst's turnover rates and overall reaction yields. Secondly, vs^hen the
catalyst is immobilized, its recovery is trivial. Therefore, if the catalyst is still active, the
film can be recycled and used in another catalytic reaction. With these motivations,
reactions using the MnP4 films, either pure or with ImODPA, were studied. Along with
the overall reaction yield, the optical intensities of the porphyrin films before and after
the reaction were monitored to determine if the porphyrins were bleached or removed
from the surface during the course of the reaction.
Battioni et al. examined the epoxidation of olefins using hydrogen peroxide as the
oxidant.! 16 in this report, Battioni investigated conditions using 5,10,15,20-tetrakis(2,6-
dichlorophenyl)porphyrin (MnTDCPP(Cl)) and 5,10,15,20-
tetrakis(pentafluorophenyl)porphyrin (MnTFPP(Cl)) with both A) excess substrate and
B) excess oxidant. The molar ratio in condition Awith MnTDCPP(Cl) was 800:10:20:1,
cyclohexene to imidazole to H2O2 to catalyst, and under these conditions, reported
cyclohexene oxide yields of 97% were observed. The molar ratio
investigated under condition B, 40:20:200:1 cyclohexene to imidazole to HjOj to
catalyst, with the same porphyrin gave epoxide yields of ca. 91%. When the catalyst
was MnTFPP(Cl), called MnPO in our studies, the yields were 58% under conditions
A and 74% under conditions B. The porphyrins containing halide substitutents on the
phenyl rings appeared to be fairly resistant to bleaching over the course of the
Baciocchi et al. studied the homogeneous epoxidation of cyclooctene to
cyclooctene oxide using both manganese and iron porphyrins as catalysts, comparing
the effects of electron donating vs. electron-withdrawing substituents on the
tetraphenyl rings.99 The porphyrins studied included 5,10,15,20-tetrakis(2,6-
dimethoxyphenyl)porphyrin (MnTDMeOPP(Cl)), the above MnTDCP(Cl), and
5,10,1 5,20-tetraphenylporphyrin (MnTPP(Cl)). Results indicate that MnTPP(Cl), in a
1 :40:200 ratio to substrate and hydrogen peroxide gave epoxide yields of 15% as
referenced to the initial substrate concentration. The MnTDCPP(Cl) derivative in the
same molar ratio to substrate and oxidant gave a 36% yield (as compared to 91% in
the Battioni report), and the MnTDMeOPP(Cl) catalyst gave a 78% yield.
When the oxidant was instead iodosylbenzene, or PhIO, the substrate was used
in excess with a molar ratio of 1 :400:20, porphyrin to substrate to PhIO, respectively.
The epoxide yields ranged from 73 to 80% with reference now to the oxidant. In both
the PhIO and HjOj epoxidation reactions, the bleaching effects observed with the
MnTDCPP(Cl) and MnTDMeOPP(Cl) catalysts were mild, under 5%. Only the
unsubstituted MnTPP(Cl) showed up to 10% bleaching reported over the course of the
2 hr reaction in the presence of PhIO.
Thin films of catalytic porphyrins have also been studied. Abatti et al.
investigated LB films containing an iron(III) 5, 10, 15, 20-tetrakis(tetradecyl-2-N-
pyridyl) porphyrin. With the inclusion of alkyl chains on the four-pyridine rings, clear
hydrophobic and hydrophihc regions were defined in the molecule and this amphiphile
formed monolayers on the water surface. These monolayers could be transferred onto
hydrophobic substrates placing the chromophore on the exposed surface. The
immobilization of these porphyrins saw a doubling of the epoxide yield as compared
to the same catalyst to reactant ratios in solution with PhIO as the oxidant.
Unfortunately, the transferred monolayer by itself was not stable and was removed
almost instantly from the surface in room temperature CH3CN. Abatti et al. used a
poly-vinylalcohol film to stabilize the porphyrin film for up to 4 hr.^^
As an alternative to the homogeneous reactions and coated LB films using
porphyrins as catalysts, our work focused on immobilizing active porphyrins in
independently stable film structures. As mentioned in Chapters 4 and 5, the process
involved incorporating manganese(III) 5, 10, 15, 20-tetrakis(tetrafluorophenyl-4-
octadecyloxyphosphonic acid)porphyrin (MnP4), with and without imidazole
octadecylphosphonic acid (ImODPA), into zirconium phosphonate LB and SA films.
The film structures were stable in hot CH2CI2 over 60 min and were expected to be
stable in the catalysis reactions.
The flow cells described and illustrated in Chapter 2 were employed to run
reactions using the porphyrin containing films with or without imidazole. The
different reaction mixtures were prepared and the blank and homogeneous solutions
were separated into different vials, where they were stirred for a given length of time.
The blank solution was pumped through a cell containing the film of interest for the
same amount of time. In most cases, the reactions were run for 2, 6, or 24 hr.
Because the extent of the reaction would be measured by GC, the first step in
this study involved determining the GC sensitivity factors for the cyclooctene and
cyclooctene oxide. A decane standard was used in PhIO reactions, and an o-
dichlorobenzene standard was used in the HjOj reactions. The sensitivity factors were
calculated using Equation 2.2:
and were found to be 1 .297 for cyclooctene oxide (CyOO) and 1 .008 for cyclooctene
(CyCO) with decane. After average k values were obtained for the CyO and CyOO,
the catalysis yields were determined from a GC of the reaction mixture using Equation
2.3. The amount of product was calculated in grams, and referenced to the theoretical
mass to obtain a percent yield. An empirical sensitivity factor for each compound is
necessary due to the fact that the GC detector does not respond identically to each
6.2. 1 Catalvsis with PhlO as the oxidant
188.8.131.52. Time dependence of oxidation yields . A stock solution of CyO and
PhIO with decane was prepared in CHjClj solution at a 1000: 125:500 molar ratio in
CH2CI2. A 1 mL aliquot contained 40 ^mol CyO, 5 ^mol PhIO and 20 fimol of
decane, which was then used for the blank reactions with no porphyrin present.
Considering the amount of porphyrin in the films to be approximately 1x10"' moles,
the molar ratio of the corresponding homogeneous reaction was .001 :40: 5:20 in, again,
1 mL of CH2CI2. Also, ca. 1 mL of the stock solution was introduced into the flow
cells containing the MnP4 film. The films used in these experiments were prepared by
self-assembling a monolayer of MnP4 onto a zirconated ODPA template and rinsing
this template with hot CHjClj. The UV-vis band, Va, which has previously been
assigned to the free-base porphyrin, is clearly present in the films used for the 24 hr
and 2 hr experiments.
When these reactions were allowed to proceed over 24 hours, GC results
indicated that the average yield in the blank was ca. 1.5 %, in the homogeneous
reaction was 8.4%, and in the films, between 20% and 35%. These percent yields, as
calculated relative to the internal standard peak, were fairly consistent.
After rinsing CHCI3
Figure 6. 1 : SA MnP4 film before and after 24 hr. catalysis run with 40:5:20
cyclooctene: PhIO:decane in CHjClj.
Interesting to note is the fact that, even after 24 hr in the catalysis reaction,
there does not appear to be a significant bleaching effect. The porphyrin appears to
still be present and intact in the film. What is noticeable is the fact that the Soret Band
has reverted back to its peak energy at 460 nm. From the known mechanism of this
epoxidation reaction, the porphyrin loses its axial ligand and becomes oxidized;
therefore, there is evidence for this ligand exchange, which is witnessed in the UV-vis
of the films after the catalysis (Figure 6.1).
When this same reaction was run for only 2 hr, the overall yields, as expected,
decreased (Table 6.1). These films were prepared in the same manner, as were the
films studied in the previous catalysis experiment. The ratio of Bands V:V1 changes
slightly, as the axial ligand environment is not identical in both cases. Again, the UV-
vis behavior before and after the catalysis reaction was studied and is shown in Figure
350 400 450 500 550
Figure 6.2: SA MnP4 film before and after 2 hr. catalysis run with 40:5:20
cyclooctene: PhIO:decane in CHjClj.
Considering the original UV-vis spectra of the films indicated in the 2 hr and
24 hr experiments, there appeared to be a significant contribution fi-om fi-ee-base
porphyrin in these films. From the studies described in Chapter 4 and 5, it was known
that self-assembling the MnP4 from a solution containing chloride ions helped
discourage the phosphonate from demetallating the porphyrin. To test if a decrease in
the film loading of free-base porphyrin affected the catalytic results, a film SA from a
chloride containing solution was employed for the 6 hr experiment. Again, some
bleaching of the porphyrin was observed, but it did appear that there was a significant
amount of the original chromophore present after catalysis. Also, there was no
significant contribution from fi-ee-base porphyrin to the UV-vis spectra even after the
catalysis run, so the catalysis does not cause the Mn-porphyrin to demetallate (Figure
6.3). However, the epoxide yield was not dramatically improved by eliminating the
free-base porphyrin from the films.
■ ■ V •
' 1 '
— After SA
- - rinsed CH^C!^
' r •
— 1 — ' —
• 1 '
350 400 450 500 550
Figure 6.3: UV-vis of MnP4 film SA from chloride containing solution used in
catalysis with PhIO after 6 hr.
The behavior of the MnP4 films in the epoxidation reaction with PhIO over 6
hr showed up to a 20% reduction in the absorbance intensity of the peak at 464 nm.
The film could be recycled with ca. a 50% loss in catalytic activity, but catalysis could
still be observed.
For comparison, the bleaching of the porphyrin in the homogeneous solution
was studied (Figure 6.4). It appears that after 6 hr., there was as much as a 50%
bleaching of the active chromophores in solution. After 12 hr, the bleaching
observed was about the same as at 6 hr. The porphyrins immobilized in the zirconium
phosphonate films appear to be slightly more stable under most catalysis conditions.
Table 6.1 : Time dependence of epoxidation of cyclooctene using 40 |amol
cyclooctene and 5 |amol PhIO in ImL of solution. To the homogeneous reaction was
added Inmol of MnPO.
* Turnovers calculated on number of oxidation cycles
** Bleaching determined from decrease in intensity of Soret Band in UV-vis
From Table 6.1, it is clear that the conversion of CyO to CyOO is time
dependent. The best yields were usually achieved after 24 hr; however, the difference
between 6 hr and 24 hr was small in some cases. Although the absorbance intensity
prior to the reaction was roughly the same as in the case of the SA films, after 24 hr,
the overall absorbance intensity has decreased. This result indicates that a percentage
of the chromophores have been bleached or removed from the surface. Considering
studies done previously on the stability of the porphyrin containing zirconium
phosphonate SA and LB films after rinsing in hot solvents, the chromophores are
probably not removed but have likely been oxidized rendering them useless as
catalysts. However, the bleaching process was not complete and there were still a
number of catalytic porphyrins available on the surface.
after 6 hr. cataysis
T ' r
MnP0 1 x10"*M
350 400 450 500 550 600
Figure 6.4: Bleaching of MnPO in homogeneous catalysis reaction with PhlO.
■ -P it V
rinse hot CHCI3
24 hr. catalysis
Figure 6.5: MnP4 LB film before and after 24 hr catalysis reaction.
184.108.40.206. Oxidation yield dependence on film preparation method . The above
described catalysis results were observed using MnP4 SA films. Alternatively, pure
LB films of MnP4 were also used in the reaction cells. A MnP4 film was transferred
by the LB technique at 20 mN m"' where the density of chromophores was high and
aggregation was observed. The film was rinsed to remove any possibly physisorbed
chromophores present due to the compressed nature of the film. These films were then
used in the reaction flow cells over 24 hr with a 40:5:20 ratio of CyO to PhIO to
decane. The corresponding blank and homogeneous reactions gave yields of, again,
around 4% and 10% respectively; however, the yields with the films were around
15%, which is slightly lower than that seen in the case of the self-assembled porphyrin
films (Figure 6.5). The lower yield could be due to the aggregated nature of the
chromophores reducing the active porphryin surface area.
Table 6.2: Conversion of cyclooctene to cyclooctene oxide with 40 |umol cyclooctene
and 5 (amol PhIO in ImL of solution using MnP4 LB film in 24 hr.
* The amount of bleaching was difficult to determine due to the broadened Soret
Band observed prior to the catalysis reaction.
6.2. 1 .3. Oxidation vield dependence on reactant ratios . Changing the ratio of
substrate to oxidant was also investigated. Whereas the prior studies focused on a
ratio of 40:5, 60:5 and 20:5 substrate to oxidant ratios were also examined over 24 hr.
Changing the substrate to oxidant ratio resulted in yields similar to that seen for the
40:5 experiment. In the 60:5 case, the overall yield was, again, around 20%); however,
the porphyrin film experienced slightly less bleaching than observed previously with
less substrate. The 20:5 case behaved similarly to the 60:5 and 40:5 experiments, and
the yields were approximately the same (Table 6.3).
Table 6.3: Conversion of cyclooctene to cyclooctene oxide with varying cyclooctene
to PhIO ratios in ImL of solution using MnP4 SA films over 24 hr.
Experiment Cyclooctene Turnovers Bleaching
6.2. 1 .4 Oxidation dependence on presence of heterocvclic lieand . Addition of
an imidazole heterocyclic ligand is commonly reported to enhance the catalytic
behavior of Mn-porphyrins in the presence of a peroxide oxidant. However, little was
mentioned in the literature about the porphyrin response to this ligand when using PhIO
as the oxidant. The imidazole was not only expected to encourage the heterolytic
cleavage of the 0-0 bond of the peroxides but also to stabilize the oxidized manganese
state; therefore, some extent of catalysis improvement might be expected. To examine
this sensitivity, a SA ImODPA/SA MnP4 film was prepared and rinsed to remove
physisorbed chromophores. Using a 40:5 substrate to PhIO ratio, the catalysis reaction
was run over 24 hr (Table 6.4).
rinse hot CHCI3
24 hr Catalysis
350 400 450 500 550
Figure 6.6: SA ImODPA/SA MnP4 studied with PhIO for epoxidation of cyclooctene.
The CyOO yields using these mixed films were actually reduced relative to the
pure porphyrin films with PhIO. The lower yield could be due to a decreased
porphyrin concentration in the mixed films. The UV-vis spectrum of the films used
for this study demonstrated that the overall absorbance of the Soret Band prior to
catalysis was lower than that observed in the pure porphyrin film (Figure 6.6). This
result corresponded to the behavior often observed in Chapters 5 where porphyrin
substitution into a preformed capping layer resulted in a slightly lower loading than in
the pure self-assembled films.
Table 6.4: Conversion of cyclooctene to cyclooctene oxide with 40 }amol cyclooctene
and 5 ^mol PhIO in ImL of solution and in films containing imidazole.
SAImODPA/SA Cyclooctene oxide Turnovers Bleaching
MnP4 Film Yield
Homogeneous 17.0%-20.0% 850-1000
Film 15.0% 750 <20%
6.2. 1 .5. Oxidation yield dependence on cell vs. vial used for reaction . The
above results appeared encouraging, with the epoxide yield consistently greater in the
immobilized catalyst reactions run in the flow cells over the homogeneous and blank
reactions run in the vials. When blank solution mixtures were run through the
catalysis cell with blank films, surprising results were obtained. After 6 hr, the blanks
in the vials ranged in product yield fi'om 1.0% to 2.0%; however, in the flow-cells, the
epoxide yield was 6.0%-6.5%. The discrepancy between the yield in the blank cell
and in the vial was not related to a change in the overall concentration. However, the
difference was probably due to either the loss of internal standard into the flow-cell
material or the fact that the solution experiences more efficient stirring in the flow-cell
than in the vial.
Further, homogeneous reactions were also run in the flow-cell to examine if the
observed increased product yield with the catalytic films was due to the immobilized
catalyst or the act of performing the reaction in the cell. These results are shown in
Table 6.5. From a series of experiments, including GCs of a solution run after the
PhIO reaction contents were removed from the cell, it appeared that decane is retained
in the cell. The decane is even observed in the GC traces after the cell has been
thoroughly cleaned, implying that this molecule has a tendency to adsorb onto the
tubing and/or cell material and dissolves out slowly. The amount of decane seen after
the cells were cleaned is small relative to the original peak, but this may be the source
of some discrepancy observed between blanks and homogeneous reactions run in and
out of the cell.
Overall, it appears that there is an improvement observed in the catalytic
activity of the porphyrin from the homogeneous solution to that seen in the
inmiobilized films. These systems clearly need to be studied using an alternative
internal standard to potentially eliminate the discrepancy observed with leeching of the
decane into and then out of the flow cell.
Table 6.5: Comparison of blanks and homogeneous epoxidation yields in vials vs. in
the reaction cells.
Blank in vial
Blank in cell
Film in cell
6.2.2. Catalysis using H oO, as the oxidant
220.127.116.11. Oxidation with iC times less catalyst vs. other reactants (80 |j.mol
H^Ot or 40 f^mol cyclooctene vs. 1 nmol MnP) . The above-described PhIO
epoxidations were performed with Inmol of catalyst relative to 40 |j,mol of substrate
and 5 jxmol of oxidant. The catalyst in these studies is ca. 10^ times lower in
concentration than that suggested in the literature studies. The results, however,
suggest that the catalyst is still active at this concentration under these conditions.
Additionally, in the PhIO reactions, excess substrate was used to protect the catalyst
from bleaching. As mentioned in section 6.1, with peroxide oxidants, both excess
substrate and excess oxidant have been investigated.
24 hr. catalysis
350 400 450 500 550 600
Figure 6.7: SA ImODPA/SA MnP4 studied in the epoxidation of cyclooctene using
When experiments similar to the Bacciochi study with excess substrate were
reproduced using 400 fimol cyclooctene and 80 ^imol H2O2 in ImL of solution, with
25 jimol imidazole and 1 nmol of MnP added to the homogeneous reaction, very little
epoxide was detected in the blank or homogeneous reactions.^^ Fortunately, a clear
increase in the yield was observed with the immobilized porphyrins (Table 6.6). As in
the PhIO reactions, the immobilized catalyst appeared to be relatively stable toward
the reaction conditions (Figure 6.7).
Table 6.6: Conversion of cyclooctene to cyclooctene oxide with 400 |amol cyclooctene
and 80 )amol H2O2 in ImL of solution using imidazole and porphyrin.
SA ImODPA/SA MnP4 Cyclooctene oxide
Films " 12.6%, 12.8%
Homogeneous solution contains 40 |j,mol ImH and Inmol of MnPO
The epoxidation of cyclooctene was also run in the presence of excess oxidant
with approximately the same molar ratio of catalyst (10"^ vs other reactants). The ratio
examined using excess oxidant was 80 i^mol cyclooctene to 400 jamol H2O2. The
results were surprising in one aspect. After the 24 hr catalysis reaction was concluded,
the UV-vis of the film showed nearly complete demetallation of the porphyrin (Figure
. V -!■'■
350 400 450 500
Figure 6.8: SA ImODPA/SA MnP4 after rinsing and after 24 hr in catalysis reaction
with excess HjOj.
Also, the epoxide yield with the film was now around 0.5% - nearly identical to that
seen in the homogeneous and blank reactions run in vials. Interestingly, when this
reaction was run in a vial with 1 |imol of MnPO, mimicking the literature procedure,
the yield was 74%, closely resembling the yield reported with a MnTFPP CI
18.104.22.168. Oxidation with 10^ times less catalyst vs. other reactants (.8 fimol
H^Ot or .4 ^mol cvclooctene vs.! nmol MnPl Because the epoxide yield in the above
reactions may be lower than usual due to the very small amount of catalyst present
relative to reactants, an attempt was made to bring these concentrations closer to the
literature ratios of 10:1 reactant to catalyst. Additionally, this oxidant concentration
(400 lamol vs. 1 nmol catalyst) clearly causes the degradation of the porphyrin films.
Figure 6.9: SA ImODPA/SA MnP4 film with catalysis using 8 i^mol cyclooctene to
0.2 |j,mol HjOj.
The solutions were, therefore, diluted 100-fold and similar reactions were
investigated. The new reaction molar ratio was 8 ^irnol cyclooctene to 0.2 ^mol H2O2
to 1 nmol of catalyst. The concentration of catalyst in the homogeneous case was set
by the concentration in the films, which could not be increased. Unfortunately, even
with excess substrate to protect the porphyrin structure, more serious catalyst
bleaching was observed in these films than in those with PhIO (Figure 6.9). In
addition, the epoxide yield with the immobilized catalyst under these conditions was
similar to or less than that observed in the homogeneous case. The reasons behind
these results are not clear.
From the above results, it appears that the immobilization of the porphyrins does
slightly improve the catalytic efficiency of the Mn-porphyrin with PhlO as the
oxidant. An improvement is also observed in the catalyst stability in the zirconium
phosphonate films relative to the homogeneous and previous LB film examples. The
stability and easy recovery of these immobilized porphyrins is an advantage over
previous literature cases.
The improvement of the epoxide yields using H2O2 using the immobilized
porphyrin and imidazole did show promise when the molar ratio was ca. 400 |j,mol
cyclooctene to 80 j^mol HjOj to 1 nmol of catalyst. Additionally, these films appeared
to be significantly more stable over 24 hr than the homogeneous catalysts.
Unfortunately, with excess oxidant, the Mn-porphyrin in the film completely
Overall, zirconium phosphonate templates allowed the successful incorporation
of both catalytic porphyrins and imidazoles into thin film environments. The
inorganic template also introduced significant film stability and overall improved the
catalytic efficiency of the immobilized catalysts.
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Mater. 1999, 11, 965-976.
Christine M. Nixon Lee was bora in Dayton, Ohio, on Febraary 17, 1973. She
moved with her family to Mansfield, Ohio, in fifth grade, and she graduated fi-om
Lexington High School in June of 1991. Christine entered Baldwin- Wallace College
in Berea, Ohio, in September 1991 as a music major, and in the fall of 1992, she began
her chemistry major. Christine graduated from Baldwin- Wallace College summa cum
laude in June of 1995 with a B.S. in chemistry and a minor in music.
In the summer of 1995, she started studying at the University of Florida, and
joined Dr. Dan Talham's group in the spring of 1997-just in time to do her oral
qualifying exam. She married Lawrence Lee in July of 1999, and will be joining him
m New York City after she graduates in the spring of 2000 with her Ph.D. in
I certify that I have read this study and that in my opinion it conforms to acceptable
standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation
for the degree of Doctor of Philosophy.
Daniel R. Talham, Chairman
Associate Professor of Chemistry
I certify that I have read this study and that in my opinion it conforms to acceptable
standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation
for the degree of Doctor of Philosophy.
William Weltner '
Professor of Chemistry
I certify that I have read this study and that in my opinion it conforms to acceptable
standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation
for the degree of Doctor of Philosophy.
Professor of Chemistry
I certify that I have read this study and that in my opinion it conforms to acceptable
standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation
for the degree of Doctor of Philosophy.
Professor of Chemistry
I certify that I have read this study and that in my opinion it conforms to acceptable
standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation
for the degree of Doctor of Philosophy.
David P. Chynoweth \
Professor of Agricultural and Biological
This dissertation was submitted to the Graduate Faculty of the Department of
Chemistry in the College of Liberal Arts and Sciences and to the Graduate School and was
accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy.
August 2000 ^
Dean, Graduate School
< . -. 7 r.
UNIVERSITY OF FLORIDA
3 1262 08555 1777